Fix opacity of Clear brushes

[kvark]

Follow-up to #3233
Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1502585
Try push: https://treeherder.mozilla.org/#/jobs?repo=try&revision=2319bbf4ce04d1af7a785567c09d43884b8c3ca3
cc @staktrace
r? anyone (once the try is green)

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This change is 

[staktrace]

r+ if try is green

[mstange]

Does this make Clear brushes go into the transparent pass? If so, that would be unfortunate: whatever is behind a Clear brush will never be visible, so ideally we should avoid drawing it.

Clear is opaque in the sense that "whatever is under me will never reach the screen". It is not opaque in the sense "after I draw, alpha will be 1".

[kvark]

Forgot to thank @mstange for hinting about the clear brushes, and now they are saying we need a different fix :) oh well. I believe the translucent() is what the old behavior was like, and it would be nice to fix the regression first before trying to optimize it. And yes, I agree that ideally we'd want to the clear brushes to be done in the opaque pass, just with the color masked out (and only alpha written). Let's take care of this as a follow-up.

@bors-servo r=staktrace

[bors-servo]

📌 Commit 8816d43 has been approved by staktrace

[bors-servo]

⌛ Testing commit 8816d43 with merge 0c69aa5...

[kvark]

Filed #3253 for the follow-up.

[staktrace]

I don't know what bors is up to, but it looks like it decided to not wait for the test merge here, and started testing 3244 instead?

[kvark]

@staktrace I accidentally force-pushed a small update to this branch which was meant for another one, and then force-pushed the old commit back in place. Surprised that bors didn't complain at all, but it did shift the focus on another task that was in the queue, so that's normal.

@bors-servo retry

[gw3583]

Once a force push occurs, I think bors might need a new r+.

@bors-servo r=staktrace

[bors-servo]

📌 Commit 8816d43 has been approved by staktrace

[kvark]

@gw3583 the tricky thing is when a force push is done for a commit that was already approved. Bors shouldn't require another r+ there

[bors-servo]

⌛ Testing commit 8816d43 with merge e162264...

[bors-servo]

☀️ Test successful - status-appveyor, status-taskcluster
Approved by: staktrace
Pushing e162264 to master...