Auto merge of #12878 - Manishearth:clip-path, r=heycam

stylo: Support clip-path

Todo:

 - [x] `set_clip_path` (probably needs a bunch of gecko bindings for running constructors/destructors)
 - [ ] Ensure that I've ordered the coordinates correctly
 - [ ] Check that it works
 - [x] Might want to convert NS_STYLE_FILL_RULE and NS_RADIUS to enum classes

Depends on:

 - https://github.com/servo/rust-bindgen/pull/29
 - https://github.com/Manishearth/gecko-dev/compare/servo:stylo...Manishearth:clip-path

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tests/unit/style/parsing/basic_shape.rs

@@ -3,6 +3,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 use parsing::parse;
+use style::parser::Parse;
 use style::values::specified::basic_shape::*;
 
 // Ensure that basic-shape sub-functions parse as both basic shapes