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
Merge branch 'master' into sdl2

Conflicts:
	comptime.sh
update SDL2 interface to use SDL_JoystickNameForIndex
clean up SDL2 video interface code
XINPUT2 support check

reable check for x11 and check if XINPUT2 support before enable mouse grab mode
TODO: need to restore non-mousegrab code?
Merge remote-tracking branch 'ilag11111/sdl2' into sdl2

Conflicts:
	src/sdl2/i_video.c
disable x11 check
use git describe to get the tag+version
grsoftwarefog is still hardware only
Merge remote-tracking branch 'upstream/master'
SRB2 2.1.7 release