Player Spin Frame MD2 Interpolation

Player spin frames on MD2's interpolate correctly when moving slowly.
Merge https://github.com/STJr/SRB2
OpenGL Screen Flip

Screen flipping in OpenGL is no longer done by inverting the screen
polygons.
Merge branch 'master' of https://github.com/Sryder13/SRB2
restore ISO C90 support in r_opengl