Closes#2041
New API: glfwSetApplication.
Stubs emitting GLFW_FEATURE_UNIMPLEMENTED for X11, Wayland and Win32.
Cocoa implementation selects the largest image provided.
Updated the icon test to also update the application icon.
This updates to a newer version of glad2 and switches to the header-only
variant.
This also (finally) switches to the newer glad2 loader signature that
allows us to pass in glfwGetInstanceProcAddress directly.
This removes all dependencies from the GLFW test programs on the Vulkan
SDK.
It also removes support for linking the GLFW shared library (dynamic
library, DLL) against the Vulkan loader static library.