Avoid tracking `Self::UniformLocation` in slotmap

[grovesNL]

We don't have a function like delete_uniform_location unlike all the other tracked resources. So we need to find a way to drop these values automatically.

Instead of the current approach we could either:

• use something like Rc<WebGlUniformLocation> for those backends and use ownership semantics and avoid storing in slotmap
• track something like Rc<WebGlUniformLocation> inside the slotmap and cleanup any WebGlUniformLocations with a strong_count of 1 whenever get_uniform_location is called

[grovesNL]

Actually it looks like WebGlUniformLocation is Clone, so maybe we can just use that.

[theypsilon]

We just talked via email about this, I'll work on a PR addressing this bug.