GDK-Win32: Add wrapper functions for calling core wgl* functions

GDK-Win32: Add wrapper functions for calling core wgl* functions

We might be dealing with GL contexts from different threads, which have more
gotchas when we are using libepoxy, so in case the function pointers for
these are invalidated by wglMakeCurrent() calls outside of GTK/GDK, such as
in GstGL, we want to use these functions that are directly linked to
opengl32.dll provided by the system/ICD, by linking to opengl32.lib.

This will ensure that we will indeed call the "correct" wgl* functions that
we need.

This should help fix issue #5685.