lavc/libopenh264enc: set slice_mode option to deprecated
"slice mode" option seems to be unnecessary since it could be
determined by -slices/max_nal_size.
default: SM_FIXEDSLCNUM_SLICE mode with cpu-number slices.
-slices N: SM_FIXEDSLCNUM_SLICE mode with N slices.
-max_nal_size: SM_SIZELIMITED_SLICE mode with limited size slices.
Add FF_API_OPENH264_SLICE_MODE macro to remove this option after
LIBAVCODEC_VERSION_MAJOR = 59.
Signed-off-by: Linjie Fu <linjie.fu@intel.com> Signed-off-by: Martin Storsjö <martin@martin.st>