Add gimppainttool-paint.[ch], which takes care of painting during
motion events in GimpPaintTool.  Perform the actual painting in a
separate thread, so that display updates, which can have a
significant synchronization overhead, don't stall painting.

Allow specific paint tools to opt-out of a separate paint thread,
and avoid it in GimpMybrushTool, since it doesn't seem to work.

The separate paint thread can be explicitly disabled by setting the
GIMP_NO_PAINT_THREAD environment variable.