libobs: skip redundant scene renders when no source is output-filtered

[BlueCheeseburger]

Replaces #12 (re-authored commits so they show as Verified on GitHub).

Summary

• Adds output_filtered_count atomic counter to obs_core_video that tracks how many sources currently have a non-zero output_filter set
• When the counter is zero (no sources are stream/record filtered), can_reuse_mix_texture() allows the stream and record mixes to reuse the main mix's rendered texture — eliminating the 3× render cost regression
• Counter is maintained at all lifecycle sites: obs_source_set_output_filter, obs_source_destroy, obs_source_duplicate, and the scene-collection load path in obs.c

Test plan

• With no sources filtered: confirm stream/record mixes reuse main mix texture (GPU render stays ~1×)
• With at least one source set to Stream Only or Record Only: confirm stream and record mixes render independently
• Source set to Record Only should be invisible in stream output and visible in recording
• Source set to Stream Only should be invisible in recording and visible in stream output
• Toggling a source filter off (back to All) decrements counter; when last filtered source cleared, reuse kicks back in

https://claude.ai/code/session_01RLn9mhsc56Y5D3esVH9WWN

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