MD2 & Patch drawing fixes

MD2's can be translucent again.
MD2's can use sprites instead of another random texture if they have no
texture.
Patches are drawn in the correct place on non aspect correct
resolutions.
Cropped Patches are drawn.
MD2's replacing skinned mobj fix

MD2's that replace mobjs that have a skin, but the skin doesn't have an
MD2 now display.
Software Cropped Patches

Fixed the stupid error I made that was causing software to not draw
cropped patches.
OpenGL Skybox

OpenGL skyboxes.
Slight change to sky vertical offsetting.
Opaque translucent midtextures

Fixed translucent midtextures being opaque when there were multiple
lights in a sector.
Merge pull request #1 from Sryder13/master

OpenGL Fixup