Support for resize operation on X11

This adds the function ```glfwResizeWindow```, that allows to resize
windows without borders.

include/GLFW/glfw3.h

@@ -900,6 +900,15 @@ extern "C" {
 #define GLFW_EGL_CONTEXT_API        0x00036002
 #define GLFW_OSMESA_CONTEXT_API     0x00036003
 
+#define GLFW_WINDOW_LEFT 0
+#define GLFW_WINDOW_TOP 1
+#define GLFW_WINDOW_RIGHT 2
+#define GLFW_WINDOW_BOTTOM 3
+#define GLFW_WINDOW_TOPLEFT 4
+#define GLFW_WINDOW_TOPRIGHT 5
+#define GLFW_WINDOW_BOTTOMLEFT 6
+#define GLFW_WINDOW_BOTTOMRIGHT 7
+
 /*! @defgroup shapes Standard cursor shapes
  *  @brief Standard system cursor shapes.
  *
@@ -2751,6 +2760,36 @@ GLFWAPI void glfwHideWindow(GLFWwindow* window);
  */
 GLFWAPI void glfwFocusWindow(GLFWwindow* window);
 
+/*! @brief Starts a resize operation with the specified window.
+ *
+ *  This function starts a resize operation on one of the borders of the
+ *  specified window.
+ *
+ *  The borders are [GLFW_WINDOW_LEFT](@ref GLFW_GLFW_WINDOW_LEFT),
+ *  [GLFW_WINDOW_TOP](@ref GLFW_WINDOW_TOP),
+ *  [GLFW_WINDOW_RIGHT](@ref GLFW_WINDOW_RIGHT),
+ *  [GLFW_WINDOW_BOTTOM](@ref GLFW_WINDOW_BOTTOM),
+ *  [GLFW_WINDOW_TOPLEFT](@ref GLFW_WINDOW_TOPLEFT),
+ *  [GLFW_WINDOW_TOPRIGHT](@ref GLFW_WINDOW_TOPRIGHT),
+ *  [GLFW_WINDOW_BOTTOMLEFT](@ref GLFW_WINDOW_BOTTOMLEFT) and
+ *  [GLFW_WINDOW_BOTTOMRIGHT](@ref GLFW_WINDOW_BOTTOMRIGHT).
+ *
+ *  @param[in] window The window to start the resize operation.
+ *  @param[in] border One of the window borders.
+ *
+ *  @errors Possible errors include @ref GLFW_NOT_INITIALIZED and @ref
+ *  GLFW_PLATFORM_ERROR.
+ *
+ *  @thread_safety This function must only be called from the main thread.
+ *
+ *  @sa @ref window_resize
+ *
+ *  @since Added in version 3.3.
+ *
+ *  @ingroup window
+ */
+GLFWAPI void glfwResizeWindow(GLFWwindow* window, int border);
+
 /*! @brief Returns the monitor that the window uses for full screen mode.
  *
  *  This function returns the handle of the monitor that the specified window is

src/internal.h

@@ -631,6 +631,7 @@ void _glfwPlatformMaximizeWindow(_GLFWwindow* window);
 void _glfwPlatformShowWindow(_GLFWwindow* window);
 void _glfwPlatformHideWindow(_GLFWwindow* window);
 void _glfwPlatformFocusWindow(_GLFWwindow* window);
+void _glfwPlatformResizeWindow(_GLFWwindow* window, int border);
 void _glfwPlatformSetWindowMonitor(_GLFWwindow* window, _GLFWmonitor* monitor, int xpos, int ypos, int width, int height, int refreshRate);
 int _glfwPlatformWindowFocused(_GLFWwindow* window);
 int _glfwPlatformWindowIconified(_GLFWwindow* window);

src/window.c

@@ -698,6 +698,19 @@ GLFWAPI void glfwFocusWindow(GLFWwindow* handle)
     _glfwPlatformFocusWindow(window);
 }
 
+GLFWAPI void glfwResizeWindow(GLFWwindow* handle, int border)
+{
+    _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window != NULL);
+
+    _GLFW_REQUIRE_INIT();
+
+    if (border < GLFW_WINDOW_LEFT || border > GLFW_WINDOW_BOTTOMRIGHT)
+        return;
+
+    _glfwPlatformResizeWindow(window, border);
+}
+
 GLFWAPI int glfwGetWindowAttrib(GLFWwindow* handle, int attrib)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;

src/x11_window.c

@@ -43,6 +43,15 @@
 #define _NET_WM_STATE_REMOVE        0
 #define _NET_WM_STATE_ADD           1
 #define _NET_WM_STATE_TOGGLE        2
+#define _NET_WM_MOVERESIZE_SIZE_TOPLEFT      0
+#define _NET_WM_MOVERESIZE_SIZE_TOP          1
+#define _NET_WM_MOVERESIZE_SIZE_TOPRIGHT     2
+#define _NET_WM_MOVERESIZE_SIZE_RIGHT        3
+#define _NET_WM_MOVERESIZE_SIZE_BOTTOMRIGHT  4
+#define _NET_WM_MOVERESIZE_SIZE_BOTTOM       5
+#define _NET_WM_MOVERESIZE_SIZE_BOTTOMLEFT   6
+#define _NET_WM_MOVERESIZE_SIZE_LEFT         7
+
 
 // Additional mouse button names for XButtonEvent
 #define Button6            6
@@ -2091,6 +2100,53 @@ void _glfwPlatformFocusWindow(_GLFWwindow* window)
     XFlush(_glfw.x11.display);
 }
 
+void _glfwPlatformResizeWindow(_GLFWwindow* window, int border)
+{
+    int winXpos, winYpos;
+    double curXpos, curYpos;
+    XClientMessageEvent xclient;
+    memset(&xclient, 0, sizeof(XClientMessageEvent));
+    XUngrabPointer(_glfw.x11.display, 0);
+    XFlush(_glfw.x11.display);
+    _glfwPlatformGetCursorPos(window, &curXpos, &curYpos);
+    _glfwPlatformGetWindowPos(window, &winXpos, &winYpos);
+    xclient.type = ClientMessage;
+    xclient.window = window->x11.handle;
+    xclient.message_type = XInternAtom(_glfw.x11.display, "_NET_WM_MOVERESIZE", False);
+    xclient.format = 32;
+    xclient.data.l[0] = winXpos + curXpos;
+    xclient.data.l[1] = winYpos + curYpos;
+    switch (border)
+    {
+        case GLFW_WINDOW_LEFT:
+            xclient.data.l[2] = _NET_WM_MOVERESIZE_SIZE_LEFT;
+            break;
+        case GLFW_WINDOW_TOP:
+            xclient.data.l[2] = _NET_WM_MOVERESIZE_SIZE_TOP;
+            break;
+        case GLFW_WINDOW_RIGHT:
+            xclient.data.l[2] = _NET_WM_MOVERESIZE_SIZE_RIGHT;
+            break;
+        case GLFW_WINDOW_BOTTOM:
+            xclient.data.l[2] = _NET_WM_MOVERESIZE_SIZE_BOTTOM;
+            break;
+        case GLFW_WINDOW_TOPLEFT:
+            xclient.data.l[2] = _NET_WM_MOVERESIZE_SIZE_TOPLEFT;
+            break;
+        case GLFW_WINDOW_TOPRIGHT:
+            xclient.data.l[2] = _NET_WM_MOVERESIZE_SIZE_TOPRIGHT;
+            break;
+        case GLFW_WINDOW_BOTTOMLEFT:
+            xclient.data.l[2] = _NET_WM_MOVERESIZE_SIZE_BOTTOMLEFT;
+            break;
+        case GLFW_WINDOW_BOTTOMRIGHT:
+            xclient.data.l[2] = _NET_WM_MOVERESIZE_SIZE_BOTTOMRIGHT;
+    }
+    xclient.data.l[3] = 0;
+    xclient.data.l[4] = 0;
+    XSendEvent(_glfw.x11.display, _glfw.x11.root, False, SubstructureRedirectMask | SubstructureNotifyMask, (XEvent *)&xclient);
+}
+
 void _glfwPlatformSetWindowMonitor(_GLFWwindow* window,
                                    _GLFWmonitor* monitor,
                                    int xpos, int ypos,