Tags: Haggus/servo

Auto merge of servo#13208 - MortimerGoro:programinfolog, r=emilio

Fix WebGL tests & Implement WebGLRenderingContext::{validateProgram, getProgramInfoLog, disableVertexAttribArray}.

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Implement WebGLRenderingContext::{validateProgram, getProgramInfoLog} and improve WebGL testcase to show shader link and validation errors.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix servo#13199 (github issue number if applicable).

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- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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