gl_common: slightly change win32 GL 3 context creation

gl_common: slightly change win32 GL 3 context creation

The code used OpenGL 3 specific functions for querying the extension
string when the actual GL 3 context wasn't created yet. This appears to
work fine on nVidia, but could break otherwise. Remove the offending
getFunctions call and retrieve the needed function pointer manually.
(This way the wglCreateContextAttribsARB function pointer can be removed
from struct GL too.)

(Amusingly exposes a wine bug; they made the same mistake.)

Explicitly check the extension string whether the function is available,
although this probably doesn't matter in practice.

Also retrieve bit depth information on win32.