d9eaeec65476955a24f5cbb42c512a81c2517d9b  Implement a VideoToolbox H265 decoder in WebCore

Classified as not security-relevant.
Triage note: Renames WebRTCWebCoreVideoEncodersEnabled->VideoCodecsEnabled and adds a native HEVC decode path: GPUVideoDecoderVTBH265, CMVideoFormatDescriptionCreateFromHEVCParameterSets soft link, and new (carefully bounds-checked) HVCC/Annex-B parsing. New feature implementation, not a fix of an existing bug.
This commit was flagged by the signal scorer and reviewed by AI (not a human). It is listed only so the triage is auditable; there is no root-cause analysis because this is not a vulnerability fix.

Structural signals: smartptr_member_added (+6.47), bounds_guard (+3.25), null_guard (+2.59), clamp_saturate (+2.92), area:Source/WTF/ (+1.0), large_churn (-2.0)