Bug 1607419 - Avoid option.expect(&format in hot code. r=jrmuizel

Easy to miss that the slow formatting code is run unconditionally.
The remaining instances are in recording and startup code.

Differential Revision: https://phabricator.services.mozilla.com/D58920

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/031a2ee21fa6e2b2983b1619ebdf65caa2b882a6

webrender/src/prim_store/mod.rs

@@ -1080,7 +1080,7 @@ impl BrushSegment {
                 let port = frame_state
                     .surfaces[surface_index.0]
                     .render_tasks
-                    .expect(&format!("bug: no task for surface {:?}", surface_index))
+                    .unwrap_or_else(|| panic!("bug: no task for surface {:?}", surface_index))
                     .port;
                 frame_state.render_tasks.add_dependency(port, clip_task_id);
                 ClipMaskKind::Mask(clip_task_id)

webrender/src/renderer.rs

@@ -1156,7 +1156,7 @@ impl TextureResolver {
             TextureSource::External(external_image) => {
                 let texture = self.external_images
                     .get(&(external_image.id, external_image.channel_index))
-                    .expect(&format!("BUG: External image should be resolved by now"));
+                    .expect("BUG: External image should be resolved by now");
                 device.bind_external_texture(sampler, texture);
                 Swizzle::default()
             }