Re-design clip/spatial API for simplicity

[Gankra]

There's a few things I want to try here, and not sure how they'll pan out:

• Make ClipChain into a single clip, use parenting for all relationships
  • Eliminate ClipChain in favour of direct clip parenting
  • OR remove clip parenting
• If ClipChain/Clip distinction is to be preserved:
  • Make ClipId only Clips, and not ClipChains
  • Make ClipChains only contain ClipIds
  • Make items only clipped by ClipChainIds
• Pull image masks out of clips, make them into a kind of Filter (since they're similar to the Opacity filter)
• Factor out SpatialTree and ClipTree from the display list as explicit seperate structures, so that SceneBuilding doesn't need to untangle them itself
• Break StackingContexts into FilterItem, BlendItem, and Picture to make a clearer seperation of concerns..?

Properties that are True about the web, which we might be relying on and which we might want to enforce at the API level:

• All items in the same StackingContext must be part of the same ReferenceFrame (but not necessarily the same ScrollFrame..?)
• One should only ever be clipped by a clip in an ancestor stacking context
  • One should only ever be clipped by a clip in the same stacking context (ancestor clipping handled by the fact that the stacking context itself will clip you)

[Gankra]

via @mstange

maybe rephrase as: "Items in a stacking context (which are always part of the same reference frame) should only be clipped by clips in that same reference frame"

and "When a clip from an ancestor reference frame needs to be applied, this clip needs to be set on an ancestor stacking context, and then the stacking context will clip you."

[kvark]

@Gankra I think a better place to post the technical details would be in the corresponding bugzilla issue, to which we'd link from here.

[Gankra]

yes, i moved to https://bugzilla.mozilla.org/show_bug.cgi?id=1615646