Fixed|qmake: Warning about missing header files

The convenience headers DrawList(s) were not using the correct case.
Fixed|Unix|GL: Fetching GL entrypoints

When linking against libGL, only OpenGL 1.x functionality is available.
Later versions of the API (as well as extensions) need to be manually
fetched.

This commit also bases the client's old sys_opengl.h on libgui's opengl.h.
Cleanup: SDL is no longer needed (except for joystick and SDL_mixer)
Fixed|Windows|Client|GL: Order of includes
Fixed|libgui|Windows: Missing exports
Better comment
Merge branch 'oculus-rift' of https://github.com/skyjake/Doomsday-Engine into oculus-rift
Rift working on Linux (but it segfaults when leaving rift mode)