fix(vision-mixer): enforce framerate on PGM/MV outputs in GL passthrough

[srperens]

Summary

• The GPU pipeline path with gl_download=false (the default) had no capsfilter on either the PGM or multiview compositor output, so pgm_framerate and multiview_framerate properties were silently ignored
• PGM often appeared correct because downstream blocks (e.g. SDI/EFP outputs) propagated framerate caps back through negotiation, but the multiview output had no such downstream constraint and ran at the input rate
• Add a capsfilter with video/x-raw(memory:GLMemory) features in both gl_download=false branches so the configured framerate/resolution is enforced while keeping buffers in GL memory

Root cause

In vision_mixer/builder.rs, the GPU pipeline conditionally added a gldownload + capsfilter chain when gl_download=true. The gl_download=false branch went straight from compositor → tee → output queue with no caps constraint at all. With the default gl_download = false, the multiview_framerate property had no observable effect.

Test plan

• cargo check clean
• cargo build (debug) clean
• cargo build --release clean
• All 408 existing tests pass (cargo test), incl. pipeline_lifecycle_test and openapi_test
• Pre-commit (fmt + clippy + secret check) passes
• Verify on frank: build runs, MV WHEP stream actually delivers configured framerate (e.g. 24fps when multiview_framerate=24/1) — observable in chrome://webrtc-internals framesPerSecond field

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