avcodec/exr: Add a gamma flag to exr loader to avoid banding

This is needed to avoid banding artifacts when gammaing the picture.
Currently, if done with a video filter, the process is done on uints
instead of full float.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/exr: mark gama option as experimental / unsupported

Suggested-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>