app: add debug utility function gimp_create_image_from_buffer()
app: whitespace fix in gimpcanvaspen.h
app: fix gimp_image_get_active_mask() for GRAY and INDEXED modes

Need to check image->active[ALPHA_G,I] for the alpha component.
Bug 699279 - GEGL tool preview different from result on image without alpha...

gimp_image_map_apply(): don't let the filter affect the drawable
projection's alpha, because it can't affect the drawable buffer's
alpha either when finally merged.
app: add gegl:checkerboard to <Image>/Filters/Render/Pattern
Implement gimp_drawable_get_linear
app: foreground extract tool remove storing all strokes