Auto merge of #17892 - kvark:wr, r=glennw

WR multi-document update

<!-- Please describe your changes on the following line: -->
The PR updates WR version to support multiple documents (https://github.com/servo/webrender/pull/1509) but doesn't take advantage of this new feature yet.
It also makes Servo to use `DevicePixel` from WR instead of rolling out another one.

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- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because _____ no extra logic

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Cargo.lock

@@ -3089,6 +3089,7 @@ dependencies = [
  "heapsize_derive 0.1.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "selectors 0.19.0",
  "serde 1.0.8 (registry+https://github.com/rust-lang/crates.io-index)",
+ "webrender_api 0.48.0 (git+https://github.com/servo/webrender)",
 ]
 
 [[package]]
@@ -3468,7 +3469,7 @@ dependencies = [
 [[package]]
 name = "webrender"
 version = "0.48.0"
-source = "git+https://github.com/servo/webrender#283192c41743a59da87b065cbc14c659d94c90b5"
+source = "git+https://github.com/servo/webrender#1ec6fb9ae47d8356920975a4be60552608dbfa1b"
 dependencies = [
  "app_units 0.5.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "bincode 0.8.0 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -3497,7 +3498,7 @@ dependencies = [
 [[package]]
 name = "webrender_api"
 version = "0.48.0"
-source = "git+https://github.com/servo/webrender#283192c41743a59da87b065cbc14c659d94c90b5"
+source = "git+https://github.com/servo/webrender#1ec6fb9ae47d8356920975a4be60552608dbfa1b"
 dependencies = [
  "app_units 0.5.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "bincode 0.8.0 (registry+https://github.com/rust-lang/crates.io-index)",