Make the pinch-zoom API a frame message instead of a scene message.

[staktrace]

Fixes #3202.

There might be other issues that are exposed by this, not sure yet. But it's a start.

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This change is 

[nical]

I think that it makes sense to move the pinch zoom to a frame message since DisplayListFlattener::create_frame_builder doesn't seem to access it. We should probably split the DocumentView struct into things that scene building needs and things that only matter for frame building.

[staktrace]

Note that create_frame_builder does use the accumulated_scale_factor() here: https://searchfox.org/mozilla-central/rev/c9272ef398954288525e37196eada1e5a93d93bf/gfx/webrender/src/display_list_flattener.rs#212 which includes the pinch zoom value. I don't really understand the purpose of that though.

[nical]

D'oh! You are right I spoke too fast. This affects resolved_transform in SpatialNodeType::ReferenceFrame which itself looks like it isn't affecting scene building (I think?). the resolved_transform member doesn't seem to need to be persisted at all. I tried removing the member and adapting the code in SpatialNode::update_transform to use a variable on the stack instead of overwriting the removed member and it compiles.

[staktrace]

Ah yes, I remember seeing that resolved_transform thing when I was working on the parallax bug. I think the only place it's used as a member variable is in print_node and I don't know how important it is there, seems like just debugging output.

[kvark]

I tried removing the member and adapting the code in SpatialNode::update_transform to use a variable on the stack instead of overwriting the removed member and it compiles.

Should we do it first (or as a part of this PR) then before merging?

[staktrace]

Sure, if you're ok with me taking it out of the print_node code I can add it to this PR.

[staktrace]

Patches updated ^

[kvark]

Reviewed 6 of 6 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved

[kvark]

@bors-servo delegate+
(leaving a chance for a try push, if needed)

[bors-servo]

✌️ @staktrace can now approve this pull request

[kvark]

Try isn't needed (as per IRC discussion).
@bors-servo r+

[bors-servo]

📌 Commit 9c43de7 has been approved by kvark

[bors-servo]

⌛ Testing commit 9c43de7 with merge 9eda2df...

[bors-servo]

☀️ Test successful - status-appveyor, status-taskcluster
Approved by: kvark
Pushing 9eda2df to master...