Add hack to load sprites from TGA

[JayFoxRox]

This is pretty broken:

• The game uses the texture size to calculate texture coordinates or something = I found no way of raising the resolution without changing sprite sizes.
• There's probably a rather low limit on the number of TGA based sprites, so not all sprites can be replaced.
• I'm not sure if the TGA sprites get free'd correctly, as there might be a different free() function involved for them.

So in order to make this any useful, more work has to be done (which can be in a follow-up PR).