Metal ICBs for MultiDrawIndirect

[creijon]

Description of Change(s)

Implementation of Indirect Command Buffers (ICBs) to improve performance of multidraw indirect on the Metal implementation of the Storm renderer.

This brings significant savings on CPU time for complex scenes.

ICBs are encoded in a transform-feedback shader which is run as part of the Prepare phase of the PipelineDrawBatch. This generates a command state, held in the HgiMetalIndirectCommands structure, which contains the ICB, argument buffers, graphics pipeline and resource bindings necessary to execute the commands in the ExecuteDraw phase of the draw batch.

The implementation add a manager to HGI, the IndirectCommandEncoder. This is responsible for the lifetime of the transform feedback shaders, interfacing with the underlying Metal API and holding a pool of buffers to optimise the creation of the ICBs.

This includes the changes submitted in the previous PRs:

• [HGI] Cleanup of vertex bindings, split off HgiMetal step functions #1876
• [HdSt] Remove excess parameters, change vega fix to member, support caps in pso #1859
• [HGI] Consistent naming of capabilities bits enum #1858

Known Issues(s)

• On Apple Silicon machines running macOS before 12.3, a driver issue causes validation failure when the supportIndirectCommandBuffers flag is set. Therefore ICBs are not supported on these versions of macOS on Apple Silicon.

Fixes Issue(s)

• CPU performance on Metal for complex scenes.

• I have submitted a signed Contributor License Agreement

[sunyab]

Filed as internal issue #USD-7426