Add support for feature level 10_0

[smoogipoo]

Do not merge

I'm making this branch both as a tracking branch for our own fork where we use feature level 10_0 to target older devices like 9800gt, and as a place to discuss how this could be mainlined if at all.

For example, should I track the selected feature level to retarget the shader version?

[frenzibyte]

A major limitation in cs_4_0 is the inability to bind RWTexture2D UAVs (known as "typed UAVs", although RWStructuredBuffer is still supported), and the inability to bind more than one UAV resource. This should be handled somewhere in Veldrid (probably as a feature in GraphicsDeviceFeatures).

See https://learn.microsoft.com/en-us/windows/win32/direct3d11/direct3d-11-advanced-stages-compute-shader#using-compute-shader-on-direct3d-10x-hardware for more information.

It would be quite nicer to use newer shader profiles if the device supports it, as that would save from doing unnecessary work to pass a UAV buffer instead of a texture (if I'm understanding device initialisation correctly, this should be as easy as doing featureLevel >= 11.0 ? cs_5_0 : cs_4_0).