v4l2: log error_idx + failing ctrl id on S_EXT_CTRLS failure

Better diagnostic when VIDIOC_S_EXT_CTRLS returns < 0: read
back error_idx and print which control id rejected (or
"ioctl-level" when error_idx == count, meaning the rejection
was generic, not per-control).

Made it possible to triage the daedalus_v4l2 phase 8.13 issue
by separating "the actual stateless control failed" (would
show failing_ctrl_id=0xa40a2c VP9_FRAME) from "libva probing
H264/HEVC profile/level we don't expose" (failing_ctrl_id=
0xa40900 H264_PROFILE etc.) — the latter is harmless on a
VP9-only context.

Before:
  v4l2-request: Unable to set control(s): Invalid argument

After (per-control):
  v4l2-request: Unable to set control(s): Invalid argument
                (error_idx=0/2 failing_ctrl_id=0xa40900 size=0)

After (ioctl-level):
  v4l2-request: Unable to set control(s): Invalid argument
                (error_idx=2/2 ioctl-level)

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

src/v4l2.c

@@ -476,12 +476,35 @@ int v4l2_set_controls(int video_fd, int request_fd,
 		      struct v4l2_ext_control *control_array,
 		      unsigned int num_controls)
 {
+	struct v4l2_ext_controls controls;
 	int rc;
 
-	rc = v4l2_ioctl_controls(video_fd, request_fd, VIDIOC_S_EXT_CTRLS,
+	memset(&controls, 0, sizeof(controls));
-				 control_array, num_controls);
+	controls.controls = control_array;
+	controls.count = num_controls;
+	if (request_fd >= 0) {
+		controls.which = V4L2_CTRL_WHICH_REQUEST_VAL;
+		controls.request_fd = request_fd;
+	}
+
+	rc = ioctl(video_fd, VIDIOC_S_EXT_CTRLS, &controls);
 	if (rc < 0) {
-		request_log("Unable to set control(s): %s\n", strerror(errno));
+		/* error_idx is the index of the first failing control;
+		 * if it equals count, the ioctl itself failed (not a
+		 * specific control payload).  Useful for triaging
+		 * which V4L2_CID_STATELESS_* the kernel rejected. */
+		if (controls.error_idx < num_controls)
+			request_log("Unable to set control(s): %s "
+				    "(error_idx=%u/%u failing_ctrl_id=0x%x size=%u)\n",
+				    strerror(errno),
+				    controls.error_idx, controls.count,
+				    control_array[controls.error_idx].id,
+				    control_array[controls.error_idx].size);
+		else
+			request_log("Unable to set control(s): %s "
+				    "(error_idx=%u/%u ioctl-level)\n",
+				    strerror(errno),
+				    controls.error_idx, controls.count);
 		return -1;
 	}