g++ warning fix
physics fix: clean up invisible cubes

have the physics engine take the same shortcuts as the render engine
fix rendering of higher mips at the edges of a map

fixes some assumptions of the engine about the rendering
 of mipmapped solid areas at the edge of the world
(confirm bug with an 8x8 solid in the corner of a /newmap)
Merge branch 'master' of https://github.com/ac-stef/AC into ac-stef-master