Add ability to check GLFW_TRANSPARENT hint outcome with

`glfwGetWindowAttrib()`

To make this possible, in situations where GLFW_TRANSPARENT is requested
but failed to match with a FBConfig, the internal framebufer hint is set
to GLFW_FALSE.

glfwGetWindowAttrib() then relies on the internal
`_glfw.framebuffer.hint.transparent` bool.

docs/window.dox

@@ -289,7 +289,9 @@ constraint.  Possible values are `GLFW_TRUE` and `GLFW_FALSE`.
 
 @anchor GLFW_TRANSPARENT
 __GLFW_TRANSPARENT__ specifies whether the framebuffer will support transparency
-in the background. Possible values are `GLFW_TRUE` and `GLFW_FALSE`.
+in the background. If transparency is not provided by your driver, GLFW will
+gracefully fallback to opaque. It is possible to check for this outcome
+with `glfwGetWindowAttrib()`. Possible values are `GLFW_TRUE` and `GLFW_FALSE`.
 
 
 @subsubsection window_hints_mtr Monitor related hints

src/context.c

@@ -318,6 +318,12 @@ const _GLFWfbconfig* _glfwChooseFBConfig(const _GLFWfbconfig* desired,
         }
     }
 
+    if (desired->transparent && closest->transparent != GLFW_TRUE)
+    {
+        // If we couldn't find a transparency match,
+        // update the fb hint for client checking.
+        _glfw.hints.framebuffer.transparent = GLFW_FALSE;
+    }
     return closest;
 }
 

src/window.c

@@ -732,6 +732,8 @@ GLFWAPI int glfwGetWindowAttrib(GLFWwindow* handle, int attrib)
             return window->resizable;
         case GLFW_DECORATED:
             return window->decorated;
+        case GLFW_TRANSPARENT:
+            return _glfw.hints.framebuffer.transparent;
         case GLFW_FLOATING:
             return window->floating;
         case GLFW_AUTO_ICONIFY: