Bug 1612440 - Add BlobImageHandler::create_similar. r=gw

Differential Revision: https://phabricator.services.mozilla.com/D74297

gfx/webrender_bindings/src/moz2d_renderer.rs

@@ -656,6 +656,13 @@ fn rasterize_blob(job: Job) -> (BlobImageRequest, BlobImageResult) {
 }
 
 impl BlobImageHandler for Moz2dBlobImageHandler {
+    fn create_similar(&self) -> Box<dyn BlobImageHandler> {
+        Box::new(Self::new(
+            Arc::clone(&self.workers),
+            Arc::clone(&self.workers_low_priority),
+        ))
+    }
+
     fn add(&mut self, key: BlobImageKey, data: Arc<BlobImageData>, visible_rect: &DeviceIntRect, tile_size: TileSize) {
         {
             let index = BlobReader::new(&data);

gfx/wr/examples/blob.rs

@@ -133,6 +133,10 @@ impl CheckerboardRenderer {
 }
 
 impl api::BlobImageHandler for CheckerboardRenderer {
+    fn create_similar(&self) -> Box<dyn api::BlobImageHandler> {
+        Box::new(CheckerboardRenderer::new(Arc::clone(&self.workers)))
+    }
+
     fn add(&mut self, key: api::BlobImageKey, cmds: Arc<api::BlobImageData>,
            _visible_rect: &DeviceIntRect, _: api::TileSize) {
         self.image_cmds

gfx/wr/webrender_api/src/image.rs

@@ -383,6 +383,11 @@ pub trait BlobImageHandler: Send {
     /// Creates a snapshot of the current state of blob images in the handler.
     fn create_blob_rasterizer(&mut self) -> Box<dyn AsyncBlobImageRasterizer>;
 
+    /// Creates an empty blob handler of the same type.
+    ///
+    /// This is used to allow creating new API endpoints with blob handlers installed on them.
+    fn create_similar(&self) -> Box<dyn BlobImageHandler>;
+
     /// A hook to let the blob image handler update any state related to resources that
     /// are not bundled in the blob recording itself.
     fn prepare_resources(

gfx/wr/wrench/src/blob.rs

@@ -128,6 +128,10 @@ impl CheckerboardRenderer {
 }
 
 impl BlobImageHandler for CheckerboardRenderer {
+    fn create_similar(&self) -> Box<dyn BlobImageHandler> {
+        Box::new(CheckerboardRenderer::new(Arc::clone(&self.callbacks)))
+    }
+
     fn add(&mut self, key: BlobImageKey, cmds: Arc<BlobImageData>,
            _visible_rect: &DeviceIntRect, tile_size: TileSize) {
         self.image_cmds