Improved WebGL architecture.

[MortimerGoro]

See #17891 and #18114

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• ./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 _____

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This change is 

[highfive]

Heads up! This PR modifies the following files:

• @asajeffrey: components/constellation/pipeline.rs, components/constellation/lib.rs, components/constellation/Cargo.toml, components/constellation/constellation.rs
• @KiChjang: components/script/dom/webglframebuffer.rs, components/script/dom/webglshader.rs, components/script/dom/webgl_validations/tex_image_2d.rs, components/script/dom/htmlcanvaselement.rs, components/script/dom/window.rs and 24 more
• @fitzgen: components/script/dom/webglframebuffer.rs, components/script/dom/webglshader.rs, components/script/dom/webgl_validations/tex_image_2d.rs, components/script/dom/htmlcanvaselement.rs, components/script/dom/window.rs and 24 more
• @emilio: components/layout/display_list_builder.rs, components/script/dom/webglframebuffer.rs, components/script/dom/webglshader.rs, components/script/dom/webgl_validations/tex_image_2d.rs, components/layout/webrender_helpers.rs and 14 more

[highfive]

Warning

• These commits modify unsafe code. Please review it carefully!
• These commits modify gfx, layout, and script code, but no tests are modified. Please consider adding a test!

[MortimerGoro]

cc @jdm

[atouchet]

Is this the same change as #18118?

[MortimerGoro]

@atouchet Yes, I didn't see that. Closing this. Thanks for the warning ;)