Give sRGB FBO's priority over float FBO's:
Both pass through linear RGB similarly, but sRGB should be faster.
If both are enabled, the .cgp author won't need float32-only features like
packing multiple values.  This also makes testing/usage priority the same.
Add sRGB FBO logging to mirror float FBO logging.
Make LUT mipmapping consistent with FBO's (ignore it for magnification.)
Nits.
Refactor sRGB FBO handling a bit.