lavc/libopenh264enc: Add qmin/qmax support

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>