ector: move RLE shape/stroke computation to a pool of thread.

ector: move RLE shape/stroke computation to a pool of thread.

This has been a long standing plan for improving performance in rendering
vector object. Depending on the test, you will get an improvement between
10 to 35% when rendering vector based object.

We are still maintaining the Cairo backend as the default one at the moment
due to a lack of result comparison tests between the two engine. Hopefully
we should get that covered and we can all enjoy a backend that is 4 times
faster by default.