Blender Git Loki

Fix #26932: When I enable multires, and start sculpting, some parts of the mesh just disappears.

Redraw issue was caused due to different redraw rectangles used for 3d view redraw and
gathering PBVH nodes to be re-drawed. I moved redraw rect expansion with rect from
previous step into sculpt_get_redraw_rect, so now redrawing works as it was planned
some commits ago -- redraw everything to which is inside currect rectangle and rectangle
from previous stroke step -- this still prevents artifact caused by fast strokes but
mesh doesn't disappear.

Brecht, Nicholas: it's the simpliest fix i could suggest atm. I've got some more
ideas with additional node flags, but it looked more complicated for me and
made code more difficult to understand. If you could see something better (like
revert all this redraw fixes for fast strokes) please tell me.