PSD files may include a "merged", pre-composited, version of the
image (in Photoshop, this is the case when saving files with
"Maximize Compatibility" enabled; GIMP always saves the merged
image data in exported PSD files.)  This commit adds support for
loading the merged image version from PSDs, instead of the full
layer hierarchy.  This is useful when loading PSD files that use
features that we don't currently support, and therefore can't
render correctly, such as adjustment layers.

When loading the merged image version, we avoid loading certain
additional data from the file, such as channels, paths, and
guides, while still loading metadata, making this akin to loading
other "flat" image formats.

This option is currently exposed as an additional file type
("Photoshop image (merged)"), which has to be explicitly selected
from the file-type list when opening the image.

(cherry picked from commit 1d9a8a91)