Add primitive interning for YUV images.

[gw3583]

Also restructure and tidy up some of the code that handles
segment building + interned primitives. This reduces some of
the code duplication and will simplify porting the remaining
primitives to interning.

---

This change is 

[gw3583]

r? @kvark or anyone

This includes #3348 and #3346 - it might be easier to review this after those land.

Pending try:
https://treeherder.mozilla.org/#/jobs?repo=try&revision=c12f066f054dc542f3b726426f1de41c1e7859de

[gw3583]

Rebased this on top of the recent patches that have landed, so there is just one commit here now.

[kvark]

, looks fairly mechanical

Reviewed 5 of 5 files at r1.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @gw3583)

---

webrender/src/prim_store.rs, line 553 at r1 (raw file):

        color: ColorF,
    },
    YuvImage {

similarly, it's unfortunate that this is duplicated with PrimitiveKeyKind

---

webrender/src/prim_store.rs, line 3214 at r1 (raw file):

        };

        debug_assert!(segment_instance_index != SegmentInstanceIndex::INVALID);

do we really need the UNUSED variant here as opposed to Option<>?

[kvark]

@bors-servo r+

[bors-servo]

📌 Commit d3187bf has been approved by kvark

[bors-servo]

⌛ Testing commit d3187bf with merge f3e489e...

[bors-servo]

☀️ Test successful - status-appveyor, status-taskcluster
Approved by: kvark
Pushing f3e489e to master...