macOS: normalize SHM names and propagate blob creation failures

[jovemexausto]

Summary

• Normalize POSIX shared-memory names on macOS before calling shm_open().
• Propagate the return value from stream_renderer_create_blob() instead of always reporting success.

Why

These fixes make blob-backed GPU resources fail loudly and keep macOS SHM creation compatible with the expected POSIX naming rules.

Validation

• Rebuilt gfxstream locally on macOS.
• Exercised this path as part of the macOS/libkrun Android integration work.

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[jovemexausto]

Reopened after signing the CLA. I hope this fix is useful to the project; it was essential for my current research, and I’m happy to help with any follow-up.

[jovemexausto]

The companion patch in libkrun is here. Together they unblock the macOS gfxstream blob-mapping path.