Initial Steps OffscreenCanvas API

[maharsh312]

@Chiaggs @PrayaniSingh0106

Initial Steps for OffscreenCanvas API

r? @jdm

• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes fix #__ (github issue number if applicable).

• There`are tests for these changes OR
• These changes do not require tests because _____

---

This change is 

[highfive]

Thanks for the pull request, and welcome! The Servo team is excited to review your changes, and you should hear from @jdm (or someone else) soon.

[highfive]

Heads up! This PR modifies the following files:

• @asajeffrey: components/script/dom/webidls/OffscreenCanvasRenderingContext2D.webidl, components/script/dom/mod.rs, components/script/dom/webidls/OffscreenCanvas.webidl, components/script/dom/offscreencanvas.rs, components/script/dom/offscreencanvasrenderingcontext2d.rs
• @jgraham: tests/wpt/include.ini
• @KiChjang: components/script/dom/webidls/OffscreenCanvasRenderingContext2D.webidl, components/script/dom/mod.rs, components/script/dom/webidls/OffscreenCanvas.webidl, components/script/dom/offscreencanvas.rs, components/script/dom/offscreencanvasrenderingcontext2d.rs

[highfive]

Warning

• These commits modify unsafe code. Please review it carefully!

[jdm]

This can use let context = OffscreenCanvasRenderingContext2D::new(self.global(), self, size); instead.

[maharsh312]

@jdm This statement give error as offscreencanvas does not have it's own global defined.
Error:"error[E0599]: no method named global found for type &dom::offscreencanvas::OffscreenCanvas in the current scope
"

[maharsh312]

Any other alternatives?

[jdm]

This doesn't compile, right? Rather than duplicating all of the canvas implementation code, I wanted to move the relevant implementation of CanvasRenderingContext2D methods to a structure that can be shared by both CanvasRenderingContext2D and OffscreenCanvasRenderingContext2D. For example:

struct CanvasState {
    saved_states: ...,
    ...
}
impl CanvasState {
    fn save(&self) {
        ....
    }
}

impl OffscreenCanvasRenderingContext2DMethods for OffscreenCanvasRenderingContext2D {
    fn Save(&self) {
        self.canvas_state.save()
    }
}

impl CanvasRenderingContext2DMethods for CanvasRenderingContext2D {
    fn Save(&self) {
        self.canvas_state.save()
    }
}

[bors-servo]

☔ The latest upstream changes (presumably #22392) made this pull request unmergeable. Please resolve the merge conflicts.

[maharsh312]

Closed the pull resquest as I am updating my GitHub fork.