Background colors should default to .clear not .white.

[willspurgeon]

Slight change to the prepareForReuse functions for LabelBrick and ButtonBrick. The background colors should default to .clear instead of .white.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@8d815f0). Click here to learn what that means.
The diff coverage is 100%.

@@           Coverage Diff            @@
##             master    #193   +/-   ##
========================================
  Coverage          ?   93.5%           
========================================
  Files             ?      40           
  Lines             ?    4449           
  Branches          ?     363           
========================================
  Hits              ?    4160           
  Misses            ?     288           
  Partials          ?       1

Impacted Files	Coverage Δ
Source/Bricks/Button/ButtonBrick.swift	86.36% <100%> (ø)
Source/Bricks/Label/LabelBrick.swift	85.18% <100%> (ø)

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