Describe the pull request

This PR adds support for view instancing/multi-view, which is particularly useful for stereoscopic rendering. Fundamentally, it allows to execute individual draw calls over multiple viewports/scissors and/or render targets. To use it, GraphicsDeviceFeatures::ViewInstancing must be enabled when creating the device. Afterwards, render passes can be created with an additional viewMask parameter. This mask is a bit set, that enables views starting with the least significant bit. Currently all implementations only support up to 4 view instances, but the engine itself does not enforce this limit direclty. As an example, the view mask 0b1111 would render to all 4 views, while view mask 0b1001 would only render into views 0 and 3. From the shader side, the current view can be accessed using the SV_ViewID semantic.

It is also possible to render into different render targets this way. This is done by outputting SV_RenderTargetArrayIndex from a pre-rasterization shader stage (i.e., vertex, geometry, tessellation or mesh). A similar semantic SV_ViewportArrayIndex is available to select the viewport and scissor (set by ICommandBuffer::setViewports and ICommandBuffer::setScissors).

Related issues

• Resolves Add support for view instancing. #183