libavodec/exr : add support for Y and YA file (ticket #5621)

a gray channel in exr, is named Y
we admit that the file need to be interpreted as gray
only if no other channel match (except alpha)

to manage RGB and Y in the color conversion part of decode_block,
the color processing is now made with a for loop.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
