Store Option<ImageInfo> instead of making fields of ImageInfo optional

[jdm]

The code in WebGLTexture stores an array of ImageInfo objects. Each of these objects has a is_initialized field, and other fields that are given default values. This makes the code harder to reason about; it would be easier to read if the array contained Option<ImageInfo> instead and any checks for initialization checked for is_none() or is_some() instead.

Code: components/script/dom/webgltexture.rs
Tests: ./mach test-wpt tests/wpt/webgl/tests/conformance/textures (there should be no difference before and after)

[highfive]

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[teapotd]

@highfive assign me

[highfive]

Hey @teapotd! Thanks for your interest in working on this issue. It's now assigned to you!