Player Spin Frame MD2 Interpolation

Player spin frames on MD2's interpolate correctly when moving slowly.
Compile Win32 EXE with -gdwarf-2
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
restore ISO C90 support in r_opengl
use git describe to get the tag+version
Merge remote-tracking branch 'Sryder13/master'
grsoftwarefog is still hardware only
use git describe to get the tag+version
grsoftwarefog is still hardware only
Merge remote-tracking branch 'upstream/master'
SRB2 2.1.7 release
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'Sryder13/master'
Render skybox from proper place for cutaway view

(Disclaimer: I don't know C at all)
Merge remote-tracking branch 'RedEnchilada/master'
Will the real 2.1.7 release please stand up?
Merge remote-tracking branch 'upstream/master'