gdkglcontext-win32-wgl.c: Fix WGL context realization on 32-bit

Hi,

This MR is spun off from !5936 (merged) as there are some likely-driver-related issues that we might want to observe a bit before landing the other things there, but the changes in this MR is to allow GL on 32-bit Windows builds to continue to work, since the calling convention of functions passed into g_clear_pointer() will matter more for 32-bit Windows programs, in particular to GL calls.

Fixes issue #5808 (closed).

With blessings, thank you!