Autodesk: Vulkan and Metal round points

[erikaharrison-adsk]

Description of Change(s)

• Add shader fragments to assign a round sample mask on points for Hgi implementations that don't support natively rasterizing round points (all modern APIs basically).
• Was originally targeting HgiVulkan, but implemented the feature at the Hgi level, so also made the necessary changes for HgiMetal.
• Add the setting HGIVULKAN_PREFERRED_DEVICE_TYPE which takes a VkPhysicalDeviceType enum int value and can be used to select between different GPUs. I needed this to test with Intel integrated graphics, which support up to 16x MSAA (instead of my default dedicated Nvidia GPU).
• Add the setting HDX_MSAA_SAMPLE_COUNT to set the MSAA sample count. Use 1 to disable MSAA. Needed this to test different sample count results.

Link to proposal (if applicable)

• N/A

Fixes Issue(s)

• N/A

Checklist

• I have created this PR based on the dev branch

• I have followed the coding conventions

• I have added unit tests that exercise this functionality (Reference:
  testing guidelines)

• I have verified that all unit tests pass with the proposed changes

• I have submitted a signed Contributor License Agreement (Reference:
  Contributor License Agreement instructions)

[jesschimein]

Filed as internal issue #USD-10507

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[jesschimein]

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