In gimp_layer_mode_get_format(), disregard the requested composite
space when selecting the format, if the input layer mode is alpha-
only, and the requested composite mode is not UNION, since, in this
case, the layer mode doesn't combine the layer/backdrop colors, and
rather only modifies the alpha of one of them.  This allows us to
use the preferred format, avoiding gamma conversion.

This particularly improves the performance of the Eraser tool in
perceptual images.