Also made fades use core functions if they are available.

MOUSE6-8 will not be supported on SDL2 as there is no interface for them.

ONLY US KEYBOARDS SUPPORTED! THAT'S HOW WE DO IT IN AMURRICA

In some cases, the warp back to center was being detected as a mouse motion, causing all sorts of silliness with the mouse. The workaround is by only using the first motion event and ignoring every event after that, until the next call to I_GetEvent.

Fixes https://mb.srb2.org/showthread.php?t=39921 where the issue before was that when you transformed your momentum would still have you moving slowly, which could cancel out the animation.

Will also fix any other case that an mobj has a skin and a single frame.

walls not extending down when a thok barrier has top textures and both
ceilings have sky.

Avoid Z-Fighting on the depth wall with the regular walls to stop the
possibility of regular walls being made partially invisible.

Gets rid of stuff being drawn where the sky should be being drawn.

sky positioning and scaling more correct compared to software.