Improve scene building and frame building performance by reducing allocations

[mattwoodrow]

This roughly triples our FPS on the displaylist_mutate talos test, and goes a long way towards resolving #3116.

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This change is 

[zptan]

Unnecessary space before ':'

[gw3583]

Reviewed 6 of 6 files at r1.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @mattwoodrow)

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webrender/src/border.rs, line 686 at r1 (raw file):

        widths: &LayoutSideOffsets,
        scale: LayoutToDeviceScale,
        brush_segments: &mut SmallVec<[BrushSegment; 4]>,

It might be worth making the size of this 8 - that should never spill to the heap then, since there will never be more than 4 corners + 4 edge segments added.

nit: Possibly worth using a type so the constant isn't repeated, e.g:
type BrushSegmentVec = SmallVec<[BrushSegment; 8]>;

___
*[webrender/src/display_list_flattener.rs, line 299 at r1](https://reviewable.io/reviews/servo/webrender/3117#-LNDs-d_B7HmERCXdiHF:-LNDs-d_B7HmERCXdiHG:b-5g6aho) ([raw file](https://github.com/servo/webrender/blob/3721810684d9caba5da3f4211f1e2da1d3426bae/webrender/src/display_list_flattener.rs#L299)):*
> ```Rust
>         self.flatten_items(&mut pipeline.display_list.iter(), pipeline_id, LayoutVector2D::zero());
> 
>         self.prim_count_estimate -= pipeline.display_list.prim_count_estimate();
> ```

I don't quite follow why we subtract here?
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*[webrender/src/segment.rs, line 193 at r1](https://reviewable.io/reviews/servo/webrender/3117#-LNDscVB2J-z1V8xmCtw:-LNDscVB2J-z1V8xmCtx:bff7tgz) ([raw file](https://github.com/servo/webrender/blob/3721810684d9caba5da3f4211f1e2da1d3426bae/webrender/src/segment.rs#L193)):*
> ```Rust
>     }
> 
>     pub fn initialize(
> ```

Since we're re-using the segment builder now, perhaps we should add a debug bool to the struct that tracks initialization, to catch any misuse?

Something like:
 - set bool to false on construction, and after build() is called
 - set to true by initialize()
 - assert that it's true at the start of build() ?

I'm not sure if it's actually worth it, thoughts? 


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[gw3583]

Looks good! r=me once CI passes.

[gw3583]

Oh, some compile errors in release builds. Must be missing some debug attributes, I guess:

error[E0609]: no field `initialized` on type `&mut segment::SegmentBuilder`
   --> webrender/src/segment.rs:212:18
    |
212 |             self.initialized = true;
    |                  ^^^^^^^^^^^
error[E0609]: no field `initialized` on type `&mut segment::SegmentBuilder`
   --> webrender/src/segment.rs:408:28
    |
408 |         debug_assert!(self.initialized);
    |                            ^^^^^^^^^^^
error[E0609]: no field `initialized` on type `&mut segment::SegmentBuilder`
   --> webrender/src/segment.rs:549:18
    |
549 |             self.initialized = false;
    |                  ^^^^^^^^^^^
error: aborting due to 3 previous errors

[pcwalton]

Note that there is the arrayvec crate if we want to both allocate on the stack and ensure that a vector never grows beyond a certain capacity.

[gw3583]

@bors-servo r+

[bors-servo]

📌 Commit f10cd10 has been approved by gw3583

[bors-servo]

⌛ Testing commit f10cd10 with merge cc80673...

[bors-servo]

☀️ Test successful - status-appveyor, status-taskcluster
Approved by: gw3583
Pushing cc80673 to master...