Fix createTexture for R and RG format textures

[djn24]

I think I've found a bug in createTexture when the texture creation options pass format: Engine.TEXTUREFORMAT_R or _RG. Repro is in this playground https://playground.babylonjs.com/#YCY2IL#1861, I've also posted to the forum with some extra information.

This PR is my attempt to fix it by using _getRGBABufferInternalSizedFormat to get the sized format, similar to other forms of texture creation in thinEngine.ts.

(This is a recreation of PR #14404. I realised I had committed with an email address I don't use for public repos so I've recreated the PR with exactly the same changes but a different committer email. I'm really sorry about the hassle.)

[bjsplat]

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[bjsplat]

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[sebavan]

Thanks a lot