[glTF] Load scenes with UINT8 indices

[footballhead]

Load scenes that contain 8-bit index buffers. How the index buffer ends up being packed depends on the capabilities of the device:

• On Vulkan with VK_KHR_index_type_uint8 extension, the UINT8 indices can be packed into a INDEX_TYPE_UINT8 buffer. This is the optimal case as it doesn't require additional memory.
• Otherwise, the UINT8 indices need to be repacked into UINT16 indices. This comes at the cost of a larger GPU buffer.

This allows us to load more Khronos glTF-Sample-Assets like TextureCoordinateTest.

Fixes #453

[Keenuts]

Seems like the required extension is https://registry.khronos.org/vulkan/specs/1.3-extensions/man/html/VK_KHR_8bit_storage.html Promoted to core in VK 1.2. BigWheels default is Vulkan1.2, so I'd say we should probably leave the buffer as-is, and request the extension?

[footballhead]

Sg I'll look at adding UINT8 support to Geometry instead

[footballhead]

Actually I'm looking at https://registry.khronos.org/vulkan/specs/1.3-extensions/man/html/VK_KHR_index_type_uint8.html which I believe is required for VK_INDEX_TYPE_UINT8_KHR. That's what I think I need for UINT8 support in Geometry. I don't see that it's been promoted

[Keenuts]

Ahh right, index buffer, not storage buffer, sorry I missed that! Not promoted, but seems like it's widely supported (>70% on Sasha willems' DB) since it was a VK_EXT_index_type_int8 before.

[apazylbe]

I tried on three devices I have (1 software lavapipe, and 2 android) and they all support VK_EXT_index_type_uint8. According to https://vulkan.gpuinfo.org/listextensions.php support for the extension is at 62%. Would it make sense to support UINT8 through the extension, or perhaps fall back to repacking if it is not available?

[footballhead]

I like the idea of supporting the extension with this as a fallback. I'll look into how to make that happen.

[footballhead]

I've incorporated #515 and #486 into this PR so it should be ready for review

[footballhead]

@apazylbe I made some non-trivial changes that I'd like you to review.

[footballhead]

@apazylbe merge conflicts are solved