openh264enc: Add deblocking mode property
openh264enc: Add some boolean properties

Background detection, adaptive quantization and scene change detection.
openh264enc: Add slice-mode and num-slices properties

slice-mode has two useful options (more available in the API but I
didn't want to expose them incorrectly) in n-slices and auto. n-slices
uses the number of slices specified in the num-slices property. auto
uses a number of slices equal to the number of threads used.
openh264enc: Change default slice mode to auto

This calculates the number of slices to use based on the number of
threads.