gdkglcontext-win32.c: Fix running with Mesa drivers

gdkglcontext-win32.c: Fix running with Mesa drivers

Some GL drivers such as Mesa-D3D12 do not allow one to call SetPixelFormat() on
a given HDC if one pixel format has been already set for it, so first check the
HDC with GetPixelFormat() to see whether a pixel format has already been set
with the HDC, and only attempt to acquire the pixel format if one has not been
set.

This will fix running with GL/NGL on Windows using the Mesa drivers.