[OpenGL] Textures are always bound to unit 0 about veldrid HOT 5 CLOSED

I understand the problem and I've verified a basic fix. I think this warrants doing a bit of cleanup related to how texture binding is handled more broadly, because it's too messy right now and there are probably other subtle issues with how the binding state is tracked. For example, because OpenGL objects are lazily initialized, there could be some other obscure issues where initializing the textures at an unexpected time could override the cached state.

Practically, I think everything needs to go through the OpenGLTextureSamplerManager if it ever touches the texture units. On top of that, the command executor probably needs to flush the resource sets right before a draw call and not assume that the texture units have been undisturbed. That will be more expensive than what I'm currently doing, but it should be fine.

As a mitigation, I these "one-off texture usages" should bind to the upper-most texture units that are available, because those are extremely unlikely to be in use. Doing so would avoid any real conflicts 99% of the time.

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@SomeAnon42 I have a fix here: 4e25323.

I'm going to run a few more tests on it on different platforms before merging it into master, but I think it is mostly correct.

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@SomeAnon42 Are you able to test out whether your scenario is fixed by the change? I did add a regression test that should match what you were seeing: https://github.com/mellinoe/veldrid/blob/4e25323875ec2b96070aad596c4d89601c8d9883/src/Veldrid.Tests/RenderTests.cs#L425

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Yes, everything is fine now.

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@SomeAnon42 Awesome, thanks for the report and the testing.

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