TGA loader

["Matthias Clasen" <matthiasc poet de>]

I recently committed the tga loader by Nicola Girardi.

Now I found that after that commit, the ico test in test-loaders fails.
The reason is that the patch moved the ico entry in the file_formats 
to the very bottom with the consequence that all ico's will now be
treated as wbmps, since pixbuf_check_wbmp is a joke. It treats
everything starting with a 0 byte as wbmp. 

I think these check functions need to be much more restrictive to
be useful for guessing. Any pointers for file format specs so that
I could look into this ?

Matthias