async scene building breaks tile decomposition rawtests

[jrmuizel]

Running the rawtests with a patch like this:

diff --git a/webrender_api/src/api.rs b/webrender_api/src/api.rs
index 35a123bc..115534ba 100644
--- a/webrender_api/src/api.rs
+++ b/webrender_api/src/api.rs
@@ -151,21 +151,21 @@ pub struct Transaction {
     generate_frame: bool,
 }

 impl Transaction {
     pub fn new() -> Self {
         Transaction {
             scene_ops: Vec::new(),
             frame_ops: Vec::new(),
             resource_updates: ResourceUpdates::new(),
             payloads: Vec::new(),
-            use_scene_builder_thread: false, // TODO: make this true by default.
+            use_scene_builder_thread: true, // TODO: make this true by default.
             generate_frame: false,
         }
     }

gives

Shader override path: None
	hit testing test...
	retained blob images test...
	blob update test...
	blob update epoch test...
	tile decomposition...
thread 'WRRenderBackend#0' panicked at 'called `Option::unwrap()` on a `None` value', libcore/option.rs:335:21
note: Run with `RUST_BACKTRACE=1` for a backtrace.

@nical @staktrace

[staktrace]

It might be the same problem as #2686, although I wouldn't expect tile decomposition to switch an image between tiled and non-tiled? I don't really know this code, but nical said he'd look into this problem when he's back from training (so presumably tomorrow).

[nical]

I had a look and what's actually happening here is that with async transactions we apply resource updates after scene building which makes sense, but we also rely on knowing which images are tiled during scene building to decompose them into several primitives if need be.

So this isn't #2686 although i suspect it is the real issue that made us realize we can have issues with images that change tiling when they are resized (without being recreated).

I am working on a patch to move decomposition out of scene building which should fix this.

[nical]

This should be fixed now.

[nical]

I checked and this is indeed fixed.