WGL extensions

[firelizzard18]

This PR:

• Adds DescribePixelFormat to gdi32.go
• Adds wglext.go with WglChoosePixelFormatARB and WglCreateContextAttribsARB

This is the minimum needed for creating modern OpenGL contexts. There are many more wglext.h functions and constants that I did not include.

OpenGL extension methods must be retrieved with WglGetProcAddress. For this to work, the call must be made with an active WGL context. Because of that, the function pointers must be explicitly initialized with InitWglExt instead of in init. Additionally, according to the spec, the function pointers returned for one context might not be valid for a different context. Realistically, they will be valid unless the two contexts use different drivers (e.g. Nvidia vs AMD). Additionally, the extension functions are optional, so applications must check that they are actually defined before calling them, hence the Has*() bool functions.