lavc/libopenh264enc: Add qmin/qmax support

Clip iMinQp/iMaxQp to (1, 51) for user specified qp range.

If not set, leave iMinQp/iMaxQp untouched and use the values (0, 51)
initialized in FillDefault(), and the QP range would be adjusted to the
defaults inside libopenh264 library according to the iUsageType, (12, 42)
for iUsageType == CAMERA_VIDEO_REAL_TIME which is default.

<https://github.com/cisco/openh264/blob/master/codec/encoder/core/src/encoder_ext.cpp#L375>
Signed-off-by: Linjie Fu <linjie.fu@intel.com>
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/libopenh264enc.c

@@ -135,6 +135,10 @@ FF_ENABLE_DEPRECATION_WARNINGS
     param.iTargetBitrate             = avctx->bit_rate;
     param.iMaxBitrate                = FFMAX(avctx->rc_max_rate, avctx->bit_rate);
     param.iRCMode                    = RC_QUALITY_MODE;
+    if (avctx->qmax >= 0)
+        param.iMaxQp                 = av_clip(avctx->qmax, 1, 51);
+    if (avctx->qmin >= 0)
+        param.iMinQp                 = av_clip(avctx->qmin, 1, param.iMaxQp);
     param.iTemporalLayerNum          = 1;
     param.iSpatialLayerNum           = 1;
     param.bEnableDenoise             = 0;
@@ -331,6 +335,12 @@ static int svc_encode_frame(AVCodecContext *avctx, AVPacket *avpkt,
     return 0;
 }
 
+static const AVCodecDefault svc_enc_defaults[] = {
+    { "qmin",      "-1"    },
+    { "qmax",      "-1"    },
+    { NULL },
+};
+
 AVCodec ff_libopenh264_encoder = {
     .name           = "libopenh264",
     .long_name      = NULL_IF_CONFIG_SMALL("OpenH264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10"),
@@ -344,6 +354,7 @@ AVCodec ff_libopenh264_encoder = {
     .caps_internal  = FF_CODEC_CAP_INIT_THREADSAFE | FF_CODEC_CAP_INIT_CLEANUP,
     .pix_fmts       = (const enum AVPixelFormat[]){ AV_PIX_FMT_YUV420P,
                                                     AV_PIX_FMT_NONE },
+    .defaults       = svc_enc_defaults,
     .priv_class     = &class,
     .wrapper_name   = "libopenh264",
 };