Give float FBO's priority over sRGB FBO's (partially undo commit 48331e8...

...2).

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 indent nearby #ifdef/#endif
pairs to make them easier to match visually.

Side notes: I also updated the error messages for sRGB/float FBO failure to be a bit more specific, and my text editor deleted some trailing whitespace.