APPLE: OIT issue fix for MaterialX

[slingthor]

Description of Change(s)

• Explicit returns for discards for Metal shading language version less than 2_4
• Parts of our earlier MaterialX PR which couldn't be included in last release are explicitly put out in PR, for discussion and hopeful landing in next release

Fixes Issue(s)

• OIT buffer invalidly written into when the intention was to stop shader execution due to discard
• When one such asset is included, especially with MaterialX, the whole frame buffer becomes noisy and glitchy

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

• I have submitted a signed Contributor License Agreement

[jesschimein]

Filed as internal issue #USD-8614

[dgovil]

Just leaving a note here for others who hit it, that this PR fixes a lot of the corruption issues that people will see using MaterialX in usdview, where you'll see tiles of images just completely disappearing and flickering inter-frame.

Without this PR, MaterialX on usdview becomes very unusable on a Mac.

[ashwinbhat]

Thanks @slingthor it works well!!