Merge pull request #3 from grahamsellers/master

Fix calling conventions for GL functions.
More WGL fixes.

Add empty handler for wglGetPixelFormat.

On WGL, change wglGetProcAddress implementation to prefer calling the real wglGetProcAddress rather than SDL_GL_GetProcAddress. The former does not return pointers to core entry points for GL <= whatever opengl32.dll supports, whereas the latter appears to return a pointer for every function. Directly calling wglGetProcAddress more closely matches the capture and will avoid divergence (here).
Draw indirect implementation.

Initial implementation of glDrawElementsIndirect, glMultiDrawElementsIndirect, glDrawArraysIndirect and glMultiDrawArraysIndirect. This will probably only work when everything is server side. I've put a check in for client-side data here, but I'm not sure if that's all that's required.