Changed the default compression level to 2

Changed the default compression level to 2

The old default was to ask for the maximum compression level. This
is against the recommendations in libvncserver/tight.c due to excessive
CPU load. It also causes Vino 3.8.1 (still shipped with Ubuntu 16.04
LTS) to prefer the blurry JPEG compression too much - e.g. red text on
the default background in MATE terminal becomes almost unreadable.

The new default is the recommended compression level for low-color
workloads, according to libvncserver source. Also, it is the maximum
compression level that doesn't trigger the Vino bug with red text in
most cases.

Fixes issue #737.