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 on Windows using the Mesa drivers.
7 jobs for mesa.win32-3.24 in 37 minutes and 24 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #1273223
debian-meson

00:18:10

passed #1273220
fedora-autotools

00:05:47

manual #1273221
allowed to fail manual
fedora-distcheck
passed #1273222
fedora-meson

00:04:11

manual #1273225
win32-ps allowed to fail manual
msys2-mingw64-autotools
passed #1273224
win32-ps
msys2-mingw64-meson

00:05:58

 
  Docs
passed #1273226
reference

00:19:13