obs-ffmpeg: Fix svt-av1 rate control settings

For SVT-AV1, rate control is explicitly set via an "rc" option, so set
those options manually for SVT-AV1. CBR doesn't seem to be fully
supported, but it's sort of supported via CVBR (constrained variable
bitrate), which seems to be specific to each GOP size, which is about as
close to CBR as we can get.

plugins/obs-ffmpeg/obs-ffmpeg-av1.c

@@ -157,15 +157,24 @@ static bool av1_update(struct av1_encoder *enc, obs_data_t *settings)
 		av_opt_set_int(enc->context->priv_data, "row-mt", 1, 0);
 	}
 
+	if (enc->svtav1)
+		av_opt_set(enc->context->priv_data, "rc", "vbr", 0);
+
 	if (astrcmpi(rc, "cqp") == 0) {
 		bitrate = 0;
 		enc->context->global_quality = cqp;
 
+		if (enc->svtav1)
+			av_opt_set(enc->context->priv_data, "rc", "cqp", 0);
+
 	} else if (astrcmpi(rc, "vbr") != 0) { /* CBR by default */
 		const int64_t rate = bitrate * INT64_C(1000);
 		enc->context->rc_max_rate = rate;
 		enc->context->rc_min_rate = rate;
 		cqp = 0;
+
+		if (enc->svtav1)
+			av_opt_set(enc->context->priv_data, "rc", "cvbr", 0);
 	}
 
 	const int rate = bitrate * 1000;