Probably fixes a random annoying crash around that function.
(I was also suspecting renderer switching was doing other slightly weird stuff.)

Has to be done manually in Lua. Use P_CreateFloorSpriteSlope, and P_DeleteFloorSpriteSlope when done with it.

Added RF_ABSOLUTEOFFSETS and RF_FLIPOFFSETS

Adjust the sort scale of splats
Do splat sorting in R_CreateDrawNodes if either vissprite is a splat, instead of if both are

Fix papersprite scaling with spritexscale and spriteyscale in Software

- Added render flags (see r_defs.h)
- Implemented floor splats
- Drop shadow sprite rendering through render flags

Simplify rotation code when the pivot isn't in the center

Replace cvar initializers with a macro

See merge request STJr/SRB2!1178