app: add gimp_layer_mode_is_trivial()

Add a TRIVIAL layer-mode flag, and corresponding
gimp_layer_mode_is_trivial() function, which indicates if the blend
function of a given layer mode is trivial, i.e., either never
modifies the source pixels (for non-subtractive modes), or always
clears the destination pixels (for subtractive modes).

(cherry picked from commit 8adec5fb3a)

app/operations/operations-enums.h

@@ -182,7 +182,8 @@ typedef enum  /*< pdb-skip, skip >*/
   GIMP_LAYER_MODE_FLAG_COMPOSITE_SPACE_IMMUTABLE =  1 << 2,
   GIMP_LAYER_MODE_FLAG_COMPOSITE_MODE_IMMUTABLE  =  1 << 3,
   GIMP_LAYER_MODE_FLAG_SUBTRACTIVE               =  1 << 4,
-  GIMP_LAYER_MODE_FLAG_ALPHA_ONLY                =  1 << 5
+  GIMP_LAYER_MODE_FLAG_ALPHA_ONLY                =  1 << 5,
+  GIMP_LAYER_MODE_FLAG_TRIVIAL                   =  1 << 6
 } GimpLayerModeFlags;