[glTF] Load UINT8 indices into UINT16 buffer

[footballhead]

Without extensions, Vulkan only supports UINT16 and UINT32 index buffers. However, glTF allows UINT8 index buffers. We can support scenes with UINT8 indices by repacking them into a UINT16 buffer instead. This comes at the cost of a larger GPU buffer.

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

Fixes #453