wgl: Remove opengl32.mingw.def.

wgl: Remove opengl32.mingw.def.

MinGW DEF parsing is even more broken than before, and 32-bits import
libs are broken regardless one uses opengl32.mingw.def or opengl32.def.

This change removes opengl32.mingw.def and addresses the issue differently:

- link opengl32.dll with --enable-stdcall-fixup

- use the systems opengl32 import lib (libopengl32.a/opengl32.lib)
  instead of our own

This change also gets test_wgl built with MinGW (even if it's never tested),
which I used to verify this; and to not link against internal libraries.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
v2: Revert back to shared_library.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10767>