The speed and quality properties have been removed in favor of properties
closer to the upstream library.
Removing the properies from the pipeline would result into a huge
slowdown so we have to map the old values to the new ones.
According to the source code of the old vp8enc element quality maps to
(int)(63 - quality * 6.2) for min_quantizer and max_quantizer, while
speed maps to cpu-used = speed == 0 ? 0 : (speed - 1).
So set min_quantizer and min_quantizer to 13, and cpu-used to 5 based on
the above formulas.
https://bugzilla.gnome.org/show_bug.cgi?id=684206
