Use raqote for canvas 2D rendering

[pylbrecht]

---

• ./mach build -d --features raqote_backend does not report any errors
• ./mach test-tidy does not report any errors
• These changes fix #23431

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This change is 

[pylbrecht]

I openend this PR to track my progress.

[pylbrecht]

I ran into some problems while trying to implement GenericPathBuilder for raqote::PathBuilder:
Unfortunately I missed the fact that raqote::PathBuilder's methods require a &mut self when creating GenericPathBuilder. I was just paying attention to azure's code, which is happy with just &self.
I feel the solution to this is changing the GenericPathBuilder trait (and adapting the implementation for azure::PathBuilder, if want to preserve successful builds for both azure and raqote). Is there an easier way to fix this?

[jdm]

The other option is using RefCell, which can call borrow_mut to obtain a &mut value.

[pylbrecht]

I'm currently facing similar issues implementing GenericDrawTarget for raqote::DrawTarget as I did while implementing GenericPathBuilder for raqote::PathBuilder:
Some methods of raqote::DrawTarget take a &mut self, where GenericDrawTarget takes just &self.

Changing the interface of GenericDrawTarget to take &mut self results in many errors in CanvasData's methods, which themselves call methods on the drawtarget member.
It's usually in the form of the one below:

error[E0502]: cannot borrow `*self.drawtarget` as mutable because it is also borrowed as immutable
   --> components/canvas/canvas_data.rs:663:9
    |
663 |         self.drawtarget.push_clip(&self.path());
    |         ^^^^^^^^^^^^^^^^---------^^----^^^^^^^^
    |         |               |          |
    |         |               |          immutable borrow occurs here
    |         |               immutable borrow later used by call
    |         mutable borrow occurs here

error: aborting due to previous error

I fiddled with this for some time now, but I'm not getting anywhere without some help.
How do I get around this ownership issue?

[jdm]

let path = self.path();
self.drawtarget.push_clio(&path);

[pylbrecht]

I tried that before and it resulted in the same error:

error[E0502]: cannot borrow `*self.drawtarget` as mutable because it is also borrowed as immutable
   --> components/canvas/canvas_data.rs:664:9
    |
663 |         let path = self.path();
    |                    ---- immutable borrow occurs here
664 |         self.drawtarget.push_clip(&path);
    |         ^^^^^^^^^^^^^^^^---------^^^^^^^
    |         |               |
    |         |               immutable borrow later used by call
    |         mutable borrow occurs here

error: aborting due to previous error

I feel it may be easier to discuss this on IRC, so I try to hit you up in #servo this week. Currently my "contribution time" is limited to the weekends, but for the sake of not being blocked, I'm going to make some extra time for this during (your) work hours, if that's fine with you?

[pylbrecht]

Update: I pushed the latest changes (46caa12), which make some of GenericDrawTarget's methods take &mut self, so I can implement the trait for raqote (some methods of raqote::DrawTarget require &mut self).
Unfortunately this results in some ownership issues, which I wasn't able to figure out by myself.

@jdm agreed to help me out by looking over these changes, when he's free, but I guess anyone's opinion is welcome. My Rust skills are still on a beginner level, so these issues might be a no-brainer.

[jdm]

@pylbrecht I resolved the remaining errors stemming from the mutability changes, and I dealt with the warning about the raqote GenericPathBuilder implementation recursively calling itself. This required adding a bit of indirection via an Option to support the fact that raqote::PathBuilder::finish consumes the calling object, unlike azure.

[pylbrecht]

@jdm, thank you! I'll try to comprehend your changes, so I get an idea of how to approach such errors by myself in the future. Thanks again for your quick response!

[jdm]

Closing since #23936 is the most up to date version of this work.