Remove 'rebuild' variable from vertexbuffer_flush

This variable is currently somewhat pointless, I was originally going to
use it to tell the graphics subsystem to completely rebuild the internal
vertex buffers, but it would be bad/inefficient to allow that
functionality.
Remove unused graphics subsystem functions

These functions were mostly related to being able to set true fullscreen
mode -- however, this has no place for our purposes, and these functions
were just sitting empty and unused, so they should be removed.

Besides, fullscreen mode only applies to the windows operating system.
d3d11:  Fix a number of warnings
UI:  Fix 'unused param' warnings in windows code
Mark unused lambda parameter
UI:  Fix a few more warnings
DShow:  Comment unused functions until needed