Auto merge of #14075 - MortimerGoro:webgl-resize, r=MortimerGoro

Implement WebGLContext resize

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Related PR: https://github.com/servo/webrender/pull/519

Implement WebGLContext resize (canvas.width & canvas.height). I have tested:

- Fixes WebGL apps that initialize canvas size after calling canvas.getContext("webgl")
- Support WebGL apps that change the canvas size & viewport on window.onresize

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- [ ] These changes fix #__ (github issue number if applicable).

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