Speed up stats computation for the whole video.

[copybara-service]

Speed up stats computation for the whole video.
E.g. on the motion_floor_to_sky_av1_hdr10p.mp4 it went down from ~1 minute to ~7 seconds.

This is achieved in two ways:

• speed up frame decoding by using the WebCodecs API (if available) to decode frame by frame, rather than repeatedly seeking to the requested time which gets slower and slower the farther away you are from a keyframe
• speed up the stats computation itself by precomputing LUTs for math functions called many times, and a bunch of other optimizations

Detailed changes:

1. WebCodecs Hardware Video Decoding (video_decoder.ts):

   • Added sequential hardware decoding of demuxed MP4 samples via WebCodecs
     VideoDecoder, eliminating O(N^2) GOP seek times in HTML5 video.
   • Added backpressure handling via decodeQueueSize and ondequeue.
   • Added Annex E.3 HEVC and AV1 codec string builders (with O(1) bitwise
     compatibility flag reversal) and box serializers.
   • Preserved fallback to HTML5 video seek if WebCodecs is unsupported or fails.

2. Color Transfer Lookup Tables (image_stats.ts):

   • Precomputed 64K-entry Float32Array LUTs for transfer functions (HLG, PQ,
     sRGB) and HLG OOTF, replacing millions of Math.pow calls with fast table
     lookups.

3. VideoFrame & WebGL Pipeline Optimizations (image_stats.ts, app.ts):

   • Allowed ImageStats to accept VideoFrame directly, avoiding createImageBitmap
     allocations.
   • Consolidated and simplified WebGL2 resource management, reusing a shared
     context, texture, and framebuffer across frames to eliminate GPU resource
     recreation overhead.
   • Added keepEncoded: false and shared scratch buffer reuse to avoid allocating
     tens of gigabytes of temporary Float32Arrays during batch processing.
   • Optimized getInverseDistribution by reusing bin boundaries, guarded
     getPercentile against division-by-zero, and optimized averageStats math.

4. UI:

   • Throttled panel re-rendering during dynamic metadata batch calculation.

5. Tests:

   • Added unit tests for video_decoder (codec strings, bit reversal).
   • Added unit tests for image_stats (ImageBitmap, scratch buffer, averaging).