[hdSt, hdx, hgi] OIT improvements and fixes

[kohakukun]

Description of Change(s)

• Implement early fragment test directly in the shader when it is not supported by the graphics API.
• Add a new test for OIT in usdImagingGL for Packed OIT and OIT
• Fix recompilation of OIT shaders when garbage collection is called.
• Add another pass/renderPassState to the oitRenderTask to hold the translucent pass information, this makes an appropriate use of the renderPassState and avoids recompilation of the OIT shaders due to pointer counter
• Merge oitDepth and oitIndex buffer into a single interleaved buffer
• Update codeGen to support handling writable and indexed interleaved buffer
• Fix small bug when querying for a resource with an empty name
• Updated oit shaders to use the new merged buffer
• Pack the transmission, depth and color for OIT
• Pack the RGBA color originally stored in a vec4 into a uvec4
• Pack the depth and transmission into a uint where the first 8 bits store the transmission(alpha) value and the rest is reserved for the depth. This results in improving the size of the OIT buffer by a factor of 4.

For the old method:

Old hdxOitDataBuffer = 102236160 ~ 97.5MB for a resolution of 1024x780 with mssa vertex stride 16
bytesPerElement = 16 numElements = 1024 * 780 = 6389760 totalSize = 1024 * 780 * 16 = 102236160

For the new method:

New hdxOitColorBuffer = 25559040 ~ 24.3MB for a resolution of 1024x780 with mssa vertex stride 4
bytesPerElement = 4 numElements = 1024 * 780 = 6389760 totalSize = 1024 * 780 * 4 = 25559040

This is defined by the theory in: https://interplayoflight.wordpress.com/2022/06/25/order-independent-transparency-part-1/

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-11636

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