Give float FBO's priority over sRGB FBO's (half-undo commit 48331e82).

sRGB FBO's should be a fallback when both are set to true: sRGB FBO's avoid
banding, but float FBO's also avoid clipping.  Users wanting to avoid both
might prefer float FBO's, but they're not supported on as many platforms,
so sRGB FBO's are the next best thing.  Also make errors more specific and
indent nearby #ifdef/#endif pairs to make them easier to match visually.