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
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