make sure to go ahead and set the skybox model when it's set/changed in FRED
go ahead and load/set the envmap when it's set/changed in FRED
get rid of extra envmap image types, only DDS is actually supported (not sure what the hell I was thinking there)
change skybox selection to be a text entry or browse instead of only text entry
allow envmap selected in background editor to be used by FRED
make texture replacement work in FRED for ships (Mantis bug #1068)
- this mainly is to just better support SEXP based starfield bitmap changes (preloading, better in-mission stuff loading)
- also fixes index_buffer related double-free()
- done waste memory for env index buffer if env is not enabled
- address a couple of bm load/release issues and comment a little to tell why