Replace solid-color property with foreground-color and add
background-color.  Pull some common code out of each of the
pattern generating functions.  Fix many of the patterns to
use foreground-color/background-color instead of white/black.
Generated images are indentical to previously if foreground-color
and background-color are left as default.

API: GstVideoTestSrc::foreground-color
API: GstVideoTestSrc::background-color