Add a Windows EGL implementation

The first several commits here muck around with some internals of the WGL frontend, to enable some simpler pointer-based access to objects without having to go through DHGLRC or HWND or HDC, since EGL is a more object-oriented API compared to WGL. Then, the pointer-based methods are exported from the DLL.

After that, the EGL target has some compiler fixes and rearranging to be able to build for Windows and reference libgallium_wgl.dll, and minor bugfixes to work on Windows.

Lastly, a basic WGL driver for EGL is added, which can SwapBuffers for a window surface. PBuffers and pixmaps and images will come later.