In gimp:mask-components, add an "alpha" property, which controls
the masked-in alpha value in case there's no aux buffer. Set it to
0 by default, so that gimp:mask-components behaves normally in the
absence of an aux buffer (as if the aux buffer was empty). Set it
to 1 in the image's visible-mask node, to maintain the current
alpha-component visibility behavior.
This fixes incorrect results when the output bounding box of a
drawable filter is smaller than the drawable, which can lead to a
NULL aux buffer being fed to the filter's gimp:mask-components
node.
Fix gimp:mask-components to use full-oapcity value for the alpha
component when it's masked-in and there's no "aux" input, so that
the image is rendered with full opacity when the alpha channel's
visiblity is toggled off, as per bug #143315.
Add specialized versions of gimp:mask-components for 8-, 16-, and
32-bpc formats, to improve efficiency, and to preserve the contents
of masked-out components exactly.
Provide public functions for format-selection and processing, which
we'll use in the painting code, instead of reimplementing component
masking.
