It is common that the same number of calls are done in the same order, so
do not stop comparing after the first divergence. This saves a significant
number of dirty rectangle in crowded scenes, like sets cn and bi where a few
calls may differ in the middle of call sequence, but later calls would stay
the same.
As a consequence, move current-frame-has-fewer-calls handling to its own
loop.
Also, factorise and extend code appending dirty rectangles.
Also, skip conseutive identical rectangles.
Vincent Pelletier
5cbdabb69d