Storing selections and layer masks as linear grayscale, but channels
as whatever-the-layers-are caused severe problems in images with
gamma-corrected layers: when combining channels with the selection,
they would go thorugh a gamma conversion before being combined, giving
unexpected results.

This commit changes all channels to always be linear, except in 8-bit
images, where they continue to be "Y' u8", for compatibility with old
XCF files, and because linear 8-bit can't really be used in
compositing (channels can be visible too).

To fix channel -> selection combinations also for these images, add a
small hack to gimp_gegl_mask_combine_buffer() which makes sure the
to-be-combined channel's pixels are always read as-is, without any
gamma conversion. After changing channels to linear, this makes no
difference except in the 8-bit images where we need this hack.