Add GimpPaintTool::paint_[xy] members, and periodically assign the
paint core's current coords to them in
gimp_paint_tool_paint_timeout(), while the main thread and the
paint thread are synchronized, during painting.

In gimp_paint_tool_draw(), fetch the current coords for the brush
outline from the above members during painting, instead of directly
from the paint core, to avoid a race condition with the paint
thread, so that we always use the correct coordinates at the time
the paint buffer was flushed back to the drawable.