In GimpPaintTool, brush outline generation took place during
gimp_paint_tool_draw() even while painting.  This function is run
concurrently with the paint thread.  When using dynamics, this
introduced a race conidition between updating the brush mask in the
paint thread, and updating the brush boundary in the main thread.

Move brush outline generation during painting to
gimppainttool-paint.c, and perform it in the display-update
timeout, while the main thread and the paint thread are
synchronized.