Apply sRGB decompression on input BitmapTextures

This was done previously in the BitmapTexture constructor, but the
changes related to the scene parser disabled this code.
The parameter 'assumedGamma' controls what decompression is to be
applied.
Add copy constructors to Bitmap.
Pre-lecture8 commit: some .OBJ files and textures.
Merge branch 'master' of https://github.com/anrieff/trinity