These were taken out in bug 914505 and then put back in for bug 907463. Since we use OP_SOURCE, they are hypothetically unnecessary and (not hypothetically) slow. But we fail a bunch of reftests, so we need to find out why, fix them, and then remove the clears.

Turns out the bug is using OP_SOURCE rather than OP_OVER (as opposed to the actual clear). We should fix the underlying bug and change to OP_SOURCE because that is more efficient on d2d.

No longer an issue since we don't plan on landing the patch in 907463 which did the de-optimisation.