Stuff like passing "input" directly if "aux"'s opacity is 0, etc.
Used to be partly handled by normal mode, even though it applies
to other modes too.
Adjust the logic for the new compositing modes.
Add a GimpLayerModeAffectMask enum, and a corresponding
get_affect_mask() function to GimpOperationLayerMode, which
specifies which of the op's inputs, if any, are affected by the
mode, apart from the overlapping regions. Most modes affect only
the overlapping regions, but dissolve and replace also affect the
rest of the input. This information is used for determining if
the optimizations are applicable.
It is used both for blending and compositing, the repeated use of the word
BLEND in code made the logic involving both blending and compositing hard to
read.
Implement a common utility function gimp_blend_composite that uses utility
functions for implementing layer modes, with separate (possibly SIMD) optimized
loops for blending and compositing, with configured linear TRC, perceptual
gamma TRC or even using CIE Lab as the space.
