Remove IImageLoader code duplication

Fixed: there was no highlight shader for internal sprites for ZDoom effects disabled;  Fixed: highlighting of transparent walls/floors was broken

Load previews when they are accessed

Dispose the bitmap as soon as it has been uploaded to a texture. Use a 1 bit alphatest bitmap for those that have transparency in them.

Fixed an issue where grid background images were drawn upside down and moved in the wrong direction on the Y axis when scrolling

Disable devil
Support mac build (which is useless as mono doesn't support winforms on modern macOS)

Fixed a problem where negative Y scales of sidedefs would result in a wrong offset when auto-aligning