This is the second in a series of commits to refactor the color handling
in sway. This removes the duplicated color parsing code in
sway/commands/client.c. Additionally, this combines the parsing of
colors to float arrays with that in sway/config.c and introduces a
color_to_rgba function in commom/util.c.
As an added bonus, this also makes it so non of the colors in a border
color class will be changed unless all of the colors specified are
valid. This ensures that an invalid command does not get partially
applied.
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