app: add GIMP_LAYER_MODE_FLAG_SUBTRACTIVE

Most modes only modify the *color* of overlapping dest/src regions,
however, erase and color-erase may also reduce their *alpha*, i.e.,
eliminate some of the overlapping content.  Flag these modes with
the new SUBTRACTIVE flag, as they require more general compositing
code.  The next commit adds the said code.

app/operations/operations-enums.h

@@ -173,6 +173,7 @@ typedef enum  /*< pdb-skip, skip >*/
   GIMP_LAYER_MODE_FLAG_BLEND_SPACE_IMMUTABLE     =  1 << 1,
   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
 } GimpLayerModeFlags;