iter25 α-25 fix: add FRAME_MBS_ONLY to H264 dummy SPS

rkvdec_h264_validate_sps doubles height when FRAME_MBS_ONLY is unset
(field-to-frame). Dummy with 1080-height was failing validation as
2176 > 1080, returning -EINVAL silently (void-cast). Even though libva
ignores the result of v4l2_set_controls, the side effect was leaving
ctx->image_fmt at ANY → first per-frame H264_SPS still hit -EBUSY in
try_or_set_cluster → setup loop broke (Bug 4 unchanged).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/context.c

@@ -270,6 +270,14 @@ VAStatus RequestCreateContext(VADriverContextP context, VAConfigID config_id,
 				(picture_height + 15) / 16 - 1;
 			dummy_sps.profile_idc = 100; /* High */
 			dummy_sps.level_idc = 41;
+			/*
+			 * FRAME_MBS_ONLY required: rkvdec_h264_validate_sps
+			 * doubles height for non-frame-mbs-only streams to
+			 * compute frame-height from field-height. Without
+			 * this flag, dummy with (height_in_map_units+1)*16
+			 * = 1088 doubles to 2176 > coded_fmt 1080 → -EINVAL.
+			 */
+			dummy_sps.flags = V4L2_H264_SPS_FLAG_FRAME_MBS_ONLY;
 
 			dummy_ctrl.id = V4L2_CID_STATELESS_H264_SPS;
 			dummy_ctrl.ptr = &dummy_sps;