without messing with the original ASTBlendPixel function

Fix ASTBlendPixel outputting empty pixels if the background pixel was empty, BUT if the foreground pixel had no alpha at all

This reverts commit 2e27f32d.

Also puts the marker skipping in the WAD API.

Also, some minor memory efficiency fixes to both rotsprite and ordinary sprite rotations.

I was using V_GetColor in a lot of places I shouldn't have, making the game look wrong with a non-default colour profile.
Though, I left R_RainbowColormap alone.