Reimplement gimp_brush_core_color_area_with_pixmap(), which copies
the brush's dab to the paint buffer when using a pixmap brush, in
terms of gimp-gegl-loops.  This simplifies the functions,
parallelizes processing, and transparently handles float brushes.

Replace the "mode" parameter of the function with an "apply_mask"
parameter, which specifies whether to apply the brush's mask to
the dab as part of copying.  Avoid applying the mask in
GimpPaintbrush; previously, we would erroneously apply the mask
twice when using the paintbrush tool: once when copying the
dab to the paint buffer, and again when pasting the paint buffer
to the canvas.

We still apply the mask in GimpSmudge, which results in the same
double-application behavior, however, this might be less practical
to fix.