gallium: make the python scripts for format parsing not fail on new formats

gallium: make the python scripts for format parsing not fail on new formats

they won't generate any useful conversion code for some of the new formats
but at least don't assert. Also needed some more hacks so they don't generate
code for some of the new formats, as gcc was not impressed.
Also declare unused channels as void, and change the scripts to not fail if
the first channel happened to be unused.
Needs serious fixing.