When we initialize OpenGL, check whether we have OpenGL 2.0 or later; if not,
check whether we have the 'GL_ARB_shader_objects' extension, since we must be
able to support shaders if using OpenGL for GTK.
If we don't support shaders, as some Windows graphics drivers do not support
OpenGL adequately, notably older Intel drivers, reject and destroy the GL
context that we created, and so fallback to the Cairo GSK renderer, so that
things continue to run, albeit with an expected warning message that the GL
context cannot be realized.
Also, when we could not make the created dummy WGL context current during
initialization, make sure that we destroy the dummy WGL context as well.
Fixes issue #4165.
This commit attempts to split GdkWin32GLContext into two parts, one for
WGL and the other for EGL (ANGLE), and attempts to simplify things a
bit, by:
* We are already creating a Win32 window to capture display changes,
so we can just use that to act as our dummy window that we use to
find out the pixel format that the system supports for WGL. We also
use it to obtain the dummy legacy WGL context that we will always
require to create our more advanced Core WGL contexts.
* Like what is done in X11, store up the WGL pixel format or the
EGLConfig in our GdkWin32Display.
* Ensure we do not create the dummy WGL context unnecessarily.
In this way, we can successfully create the WGL/EGL contexts, however
there are some issues at this point:
* For WGL, the code successfully initializes and realizes the WGL
Contexts, but for some reason things became invisible. When running
gtk4-demo, this can be verified by seeing the mouse cursor changing
when moved to spots where one can resize the window, although they
were invisible.
* For EGL, the code initializes EGL but could not realize the EGL
context as shaders failed to compile. It seems like the shader issue
is definitely outside the scope of this MR.
