WebGL Texture2d array

[heretique]

Adds support for WebGL2 TEXTURE2D_ARRAY

I confirm I have read the contributing guidelines and signed the Contributor License Agreement.

This PR adds support for 2d texture arrays useful in a lot of scenarios but most commonly for terrain rendering.
It introduces a new uniform type UNIFORMTYPE_TEXTURE2D_ARRAY and sampler type sampler2DArray ca be used now inside the shaders.
There are also a few small additions to the options used when constructing a pc.Texture

[options.arrayLength] - The number of textures in a 2D texture array (not supported by WebGl1).
[options.levels]            - These gain an additional type Uint8Array[][]

The PR also adds a new simple example Texture Array that shows how a 2D Array texture can be created and used.
There is no WebGPU support for this at the moment.
The textures used are from https://polyhaven.com/ and are public domain (CCO license).

I'm pretty sure the PR is rough around the edges and I'm looking forward for PlayCanvas team's feedback.

[Maksims]

Is there a new public API related to this PR?
If so, could you please write it in first post?

[heretique]

@Maksims I updated the first post. I hope it is more clear now.
As a first PR to add support I believe this could be useful but it does have one big limitation, similar to the 3d textures you can only update the whole texture array.
It would be also useful to be able to update a single texture and it's mips.
That would used texSubImage3D and would be more in line with PRs like #5759 speeding up updates.

[mvaligursky]

Related issue: #4265

[mvaligursky]

Ideally Texture.lock would allow us to lock individual slice as well, but that would be done in a separate PR.

[heretique]

Ideally Texture.lock would allow us to lock individual slice as well, but that would be done in a separate PR.

Yeah, totally agree there are a lot of cases where it would make sense to be able to upload individual slices. I will add a separate PR for that as well after this is merged.

[mvaligursky]

Looks great to me. Any thoughts @willeastcott @slimbuck ?