Removed pointless unmap.

There was no buffer actually mapped here.
Fix part of issue #24.

A simple typo. This caused DXT3 and DXT5 textures to seem uncompressed, and then of course no correct format and type values could be found for them. Still, that's not all; the 32-bit non-alpha files still won't load properly.
Check for valid type and format, fail if not found.
Load a default texture if a texture file can't be loaded anymore.

I'm wondering whether I should do this for textures that can't be loaded, too.
Fixed loading of 32-bit RGB DDS files.

Lots of small stuff that didn't work. But this has fixed issue #24.