Allow storing values in intermediate framebuffers as sqrt(x).

Allow storing values in intermediate framebuffers as sqrt(x).

Together with GL_RGB10_A2, this would seem to be an even better tradeoff for
many chains than GL_SRGB8_ALPHA8 is, as long as you don't need intermediate
alpha. (We verify its accuracy with a unit test.)

This changes the API for specifying intermediate framebuffers, but that API
was never in a release, so it should be fine.

Also document a rather obscure problem where, if you can actually hold on to
non-linear values across a bounce buffer, you don't really want to store them
in sRGB encoding. (The square-root version actually avoids this problem.
I guess we could snoop on the type and do a similar thing if we see it's an
GL_SRGB* encoding, but it seems so obscure that we can ignore it for now.)