Move index format out of input state #258

In the three native APIs, the index data type is not specified until command buffer encoding time. It seems there's no reason for us to have indexFormat inside vertex input state; it can move to setIndexBuffer.

• Metal: https://developer.apple.com/documentation/metal/mtlrendercommandencoder/1515520-drawindexedprimitives?language=objc
• D3D12: https://docs.microsoft.com/en-us/windows/desktop/api/d3d12/nf-d3d12-id3d12graphicscommandlist-iasetindexbuffer
• Vulkan: https://www.khronos.org/registry/vulkan/specs/1.1/html/vkspec.html#vkCmdBindIndexBuffer