Commit f5cb1fed, which performed
brush outline generation in GimpPaintTool in synchrony with the
paint thread, wasn't enough, since GimpSourceTool could still call
gimp_brush_tool_create_outline() directly during its
GimpDrawTool::draw() method, leading to the same race condition
when executed concurrently with the paint thread.

Partially revert the above commit, so that outline generation is
handled as before, as far as GimpPaintTool is concenered.  Instead,
add GimpPaintTool::{start,end,flush}_paint() virtual functions; the
first two are called when starting/ending painting using the paint
thread, while the third is called during the display-update
timeout, while the main thread and the paint thread are
synchronized.  This allows subclasses to perform non-thread-safe
actions while the threads are synchronized.

Override these functions in GimpBrushTool, and cache the brush
boundary in the flush() function.  Use the cached boundary in
gimp_brush_tool_create_outline() while painting, to avoid the above
race condition, both when this function is called through
GimpPaintTool, and through GimpSourceTool.