Add a boolean "direct" parameter to gimp_projection_flush_now(),
which specifies if the projection buffer should only be invalidated
(FALSE), or rendered directly (TRUE).

Pass TRUE when flushing the projection during painting, so that the
affected regions are rendered in a single step, instead of tile-by-
tile.  We previously only invalidated the projection buffer, but
since we synchronously flush the display right after that, the
invalidated regions would still get rendered, albeit less
efficiently.

Likewise, pass TRUE when benchmarking the projection through the
debug action, and avoid flushing the display, to more accurately
measure the render time.