Some SDL-related compiling tweaks

* -lSDL2_mixer is already added to SDL_LDFLAGS by default, unless NOMIXER=1 is set
* -DSDLMAIN should also be added to OPTS by default for MINGW=1 builds, unless NOSDLMAIN=1 is set

Fix OpenGL Fademasks (Screen Transitions)

Screen transitions don't work in OpenGL when scr_depth is set to 8 or 16 bits.
This fixes that.

See merge request !67

For some reason every texture and flat loaded into GL is vertically flipped

Don't think either of them need RGBA

So they don't get effected by the texture format set by the screen depth
GL_RGB5_A1 from 16-bit was removing all alpha from the texture

Fix how FF_CUTEXTRA FOFs carry over cutting ability to FOFs below them in the same sector, even if they shouldn't do that

Always wondered why a flag meant for FOF physics was used in the rendering code, lol

Clang cleanup

"taking the absolute value of unsigned type 'unsigned int' has no effect"

using abs() on an unsigned should be nop,  replace them with (angle_t)()

Need someone to verify this does not break any game logic

See merge request !63