context: introduce request_pool, decouple OUTPUT buffers from surfaces

Commit 3 of the upstreamable plan (upstreamable_design.md §1, §5).
Replaces the prior per-surface OUTPUT-buffer ownership model with a
small driver-wide pool sized by codec pipeline depth (4 H.264 frames
in flight), allocated unconditionally regardless of caller's
num_render_targets.

Prior art (kernel UAPI dev-stateless-decoder.rst, ffmpeg
v4l2_request.c, Chromium V4L2StatelessVideoDecoder, GStreamer
v4l2slh264dec) all decouple OUTPUT and CAPTURE pool sizing. fourier's
"output_count == surfaces_count" model was a category error: OUTPUT
buffers are request-time bitstream slots, CAPTURE buffers are
picture-time DPB slots; their lifecycles and sizing are independent.

Changes:
  * NEW src/request_pool.{c,h} (~200 LoC):
      - request_pool_init(): CREATE_BUFS + per-slot QUERYBUF + mmap.
      - request_pool_destroy(): munmap all, idempotent.
      - request_pool_acquire(): round-robin claim; returns V4L2 buffer
        index of an unused slot or -1.
      - request_pool_release(): mark slot free for reuse.
      - request_pool_slot(): accessor for ptr/size given a buffer index.

  * src/request.h: add struct request_pool output_pool to request_data.

  * src/context.c::RequestCreateContext: replace the per-surface
    OUTPUT loop with a single request_pool_init() call (count=4,
    independent of surfaces_count). Drop the now-unused locals
    (length, offset, source_data, output_buffers_count, index,
    index_base, i, surface_object). DELETES patch 0002's
    "output_buffers_count = ... ? ... : 4" hack inline — the pool's
    own count parameter supersedes it.

  * src/picture.c::RequestBeginPicture: borrow a pool slot at frame
    start, write its mmap pointer/size/index into the surface's
    transient source_* fields. The fields stay (still useful as
    a borrow handle that the existing codec_store_buffer memcpys
    target), but no longer represent surface-permanent ownership.
    Reset slices_size/slices_count here too (was implicit on first
    Render).

  * src/surface.c::RequestSyncSurface: after VIDIOC_DQBUF returns
    the OUTPUT buffer, release the pool slot and clear the surface's
    borrow handle. Fixes the segv on second-frame submission.

  * src/surface.c::RequestDestroySurfaces: remove the munmap of
    source_data — pool owns the mmap.

  * src/request.c::RequestTerminate: call request_pool_destroy()
    before close(video_fd) so munmaps still target a valid fd.

  * src/meson.build: add request_pool.c and request_pool.h to the
    sources/headers lists.

This commit removes 0002's OUTPUT-pool hack inline (the
"floor to 4" line is gone). The DECODE_MODE/START_CODE block in 0002
remains until commit 4 lands.

Build-verified clean on aarch64.

Signed-off-by: Markus Fritsche <fritsche.markus@gmail.com>

src/meson.build

@@ -44,6 +44,7 @@ sources = [
 	'v4l2.c',
 	'mpeg2.c',
 	'h264.c',
+	'request_pool.c',
 #	'h265.c'
 ]
 
@@ -64,6 +65,7 @@ headers = [
 	'v4l2.h',
 	'mpeg2.h',
 	'h264.h',
+	'request_pool.h',
 #	'h265.h'
 ]