Support for rendering multiple viewports at once on Metal (i.e for XR)

[PixelPartner]

On twitter, Rob kindly offered assistance for non-iOS metal and asked me to open this issue. Metal already supported single pass rendering to more than one viewport on macOS for most of its x86 GPUs and with iOS12 and on the GPUFamily5 (A12) it's also supported on iOS for up to 16 viewports. https://developer.apple.com/documentation/metal/mtlrendercommandencoder/2869738-setviewports

[RobDangerous]

Wasn't refering to macOS actually but to D3D12 and Vulkan. Not sure my Mac can do it, have to check.