gimp_drawable_start/end_paint() are used to enter/exit paint mode
for a given drawable.  While the drawable is in paint mode,
gimp_drawable_get_buffer() returns a copy of the real drawable's
buffer, referred to as the paint buffer, so that modifications to
the returned buffer don't immediately affect the projection, and
calls to gimp_drawable_update() queue the updated region, instead
of emitting an "update" signal.

gimp_drawable_flush_paint() can be called while the drawable is in
paint mode, in order to copy the updated region of the paint buffer
back to the drawable's real buffer, and to emit "update" signals
for the queued region.

We use these functions in the next commit, to move painting to a
separate thread in the paint tools.