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 !1178

Provide a default exe manifest

See merge request !1174

Fix some theoretical crashes when using the P_*XYMovement, P_*ZMovement, and P_RailThinker functions in Lua.

See merge request !1173

Add error message when the GDI Generic OpenGL renderer is encountered in Windows

See merge request !1172