Make shader parsing more future-proof

[zturtleman]

We should call SkipRestOfLine( ); after done parsing every line, similar to q3map2. So if keyword gets more (optional) args in the future, old spearmint won't blow up.

q3map2 ignores lines that start with an unknown keyword. Should the renderer do this too? It would make it easier to add new keywords later, but also means keyword typo will get ignored. Could add new #if keywords (example: #if ext_musicWaveform) to check if renderer supports a new feature instead.

[zturtleman]

Done in cef4cd1.