h264: gate SCALING_MATRIX submission on VAIQMatrixBuffer presence

VAAPI signals "explicit scaling lists are present in the bitstream"
implicitly: the consumer (ffmpeg-vaapi, mpv, etc.) sends a
VAIQMatrixBufferH264 alongside RenderPicture iff
sps_scaling_matrix_present_flag || pps_scaling_matrix_present_flag.
When the bitstream uses default (flat) scaling, no IQMatrixBuffer
arrives and the in-tree h264.matrix struct stays zero-initialised.

fourier's existing codec_store_buffer for MPEG2 and HEVC tracks this
via a per-surface iqmatrix_set boolean (surface.h::mpeg2.iqmatrix_set,
h265.iqmatrix_set) — the H.264 path was missing the equivalent flag,
so set_controls always submitted the scaling matrix, including the
zero-initialised case.

Symptom on hantro-vpu RK3568: when TRANSFORM_8X8_MODE is enabled in
PPS, the kernel multiplies all 8x8 DCT coefficients by the zeroed
scaling_list_8x8, producing a zeroed CAPTURE buffer despite a
successful decode round-trip (no V4L2_BUF_FLAG_ERROR,
bytesused=3655712 reported).

Earlier draft of this patch unconditionally omitted SCALING_MATRIX in
FRAME_BASED. That's corpus-correct (bbb has no explicit scaling
lists) but the wrong predicate: the kernel-side gating is by
"matrix-supplied vs. not," not by decode mode. Streams that signal
explicit scaling lists must submit SCALING_MATRIX in either mode.

Contract verification (audit_0008_decode_params_2026-05-01.md +
hantro_h264.c::assemble_scaling_list): the kernel uses the supplied
matrix when SCALING_MATRIX is in the control batch and falls back
to spec-defined defaults when absent. Mode-independent.

This patch:
  - surface.h: adds bool matrix_set to params.h264, mirroring
    mpeg2.iqmatrix_set / h265.iqmatrix_set.
  - picture.c codec_store_buffer (H.264 VAIQMatrixBufferType case):
    sets matrix_set = true when the buffer arrives.
  - picture.c RequestBeginPicture: resets matrix_set = false at the
    start of each Begin/Render/End cycle.
  - h264.c h264_set_controls: builds the controls[] array
    incrementally; SPS/PPS/DECODE_PARAMS always; SCALING_MATRIX iff
    matrix_set; SLICE_PARAMS only in SLICE_BASED; PRED_WEIGHTS only
    when both SLICE_BASED and V4L2_H264_CTRL_PRED_WEIGHTS_REQUIRED.

The pre-existing FRAME_BASED-omits-SLICE_PARAMS rule is preserved —
kernel doc ext-ctrls-codec-stateless.rst:752: "When this mode is
selected, the V4L2_CID_STATELESS_H264_SLICE_PARAMS control shall
not be set."

Cross-reference: kernel UAPI section
ext-ctrls-codec-stateless.rst V4L2_CID_STATELESS_H264_SCALING_MATRIX
(matrix supplied iff explicit scaling lists in bitstream) and
hantro_h264.c::assemble_scaling_list (consumes supplied matrix or
falls back to defaults).

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

src/surface.h

@@ -73,6 +73,7 @@ struct object_surface {
 		} mpeg2;
 		struct {
 			VAIQMatrixBufferH264 matrix;
+			bool matrix_set;
 			VAPictureParameterBufferH264 picture;
 			VASliceParameterBufferH264 slice;
 		} h264;