vo_postprocess: add overlay module (native alpha blending, 9 codecs)

[benroeder]

Adds an overlay postprocessor that blends a PAM image onto video in
its native pixel format. Supports RGBA, RGB, UYVY, YUYV, v210, R10k,
R12L, Y416, RG48, and I420 - conversion + blend in one pass per
codec, no intermediate format. Hot-reload via mtime/size watcher,
optional libswscale resize (scale=WxH or scale=frame for resolution
tracking), soft-edge alpha fade, pthread-pool blend parallelism.
Multiple instances chain via comma in --postprocess.

Usage:

-p overlay:file=PATH[:position=POS][:custom_x=X:custom_y=Y]
          [:soft_edge=N][:scale=WxH|frame[:scale_filter=F]]
          [:blend_threads=N][:perf]

overlay:help prints the keyword reference.

Implementation:

• 16-bit RGBA internal. Blend functions use the existing RGB_TO_Y/CB/CR
  macros + get_color_coeffs() so --color-601 still works.
• YCbCr-space blend for YUV destinations.
• Async reload off the postprocess thread; postprocess polls
  task_is_done() (added to utils/worker.{h,cpp}) and harvests on the
  next frame.
• Per-row blend dispatched via task_run_parallel, threshold-gated at
  500k pixels. Default blend_threads = min(ncpu, 8).
• Oversized overlay returns src_x/src_y on the rect so the visible
  region maps to the correct slice (centre/right) instead of being
  clamped to top-left.
• scale=frame re-renders on source resolution changes, detected per
  frame in overlay_postprocess (not the reconfigure callback - some
  pipeline paths skip it).

Tests:

• Unit (bin/run_tests): per-codec blend correctness, PAM loader,
  config parser, layout, soft edge, watcher, scaler.
• E2E (test/run_overlay_e2e.sh): 19 assertions - per-codec exact-byte
  verification, threaded-blend parity (RGBA and I420), dual-overlay
  chain, oversized-centre, scale=frame stretch.
• Optional rapidcheck (./ext-deps/bootstrap_rapidcheck.sh +
  --enable-rapidcheck + make rapidcheck-tests): 33 properties x 100
  cases.
• Visual: test/run_overlay_visual.sh, test/run_scale_frame_visual.sh.
• Benchmarks: test/benchmark_overlay_*.sh.

Performance: I420 1080p overlay at 4K30 runs in 3.84 ms blend
single-threaded, 0.71 ms with 8 threads (5.4x). Other codecs scale
similarly under the same MIN_PARALLEL_PIXELS gate.

Limitations:

• BT.2020 not supported (codebase-wide gap in get_color_coeffs).
• PAM has no gamma metadata; transfer-function correction ignored
  (matches pixfmt_conv.c).
• Straight alpha only.
• scale=frame stretches without preserving aspect ratio.

Files:

• src/vo_postprocess/overlay.c
• src/utils/alpha_blend.{c,h}
• src/utils/overlay_{config,layout,pam,scale,soft_edge,watch}.{c,h}
• src/utils/worker.{h,cpp} (+task_is_done)
• test/* (unit, property, e2e, visual, benchmark)
• ext-deps/bootstrap_rapidcheck.sh, configure.ac, Makefile.in (opt-in)

No upstream files modified outside worker.{h,cpp}, configure.ac,
Makefile.in, .gitignore.