Envshot

new functionality:
 - envshot console command creates cubemeaps (env maps)
 - envToSky console command converts env maps to sky maps ( _px, _py _pz, _nx, _ny, _nz tga files to _forward, _back, _left, _right, _u, _down tga files) it applies transforms (rotations and flipping on some of the maps)
 - SkyToEnv console command converts skymaps to env maps

as well as infrastructure functions to support those new console commands

I've modified "take screenshot" so now it can take
PNG and TGA (and if ever there is a JPG handler available, it should be possible to add a call to it quite easily)

possible future improvements:

 - add scale options for the envToSky and the skyToEnv console commands.
 - add a skyShot console command that uses envshot and automatically applies envToSky

I'm not going to work on those improvements, next step for me is skipable cinematics.
fileType_t sounds so generic.. maybe imageFileType_t ?