Fix color object guideline from math glyph

[LettError]

For your consideration. This addresses issue #850.
Summary: during fromMathGlyph, a guideline can have a color that can be a defcon color object rather than a tuple. This adds some resilience without creating too many expectations or dependency on defcon.

This is a small but annoying issue that in effect requires all guidelines to be removed from source glyphs before starting calculations.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.25%. Comparing base (29dabb3) to head (704c188).
⚠️ Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
Lib/fontParts/base/glyph.py	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #851      +/-   ##
==========================================
- Coverage   78.31%   78.25%   -0.07%     
==========================================
  Files          41       41              
  Lines        6107     6112       +5     
  Branches      916      917       +1     
==========================================
  Hits         4783     4783              
- Misses       1117     1122       +5     
  Partials      207      207              

Files with missing lines	Coverage Δ
Lib/fontParts/base/glyph.py	76.89% <0.00%> (-0.40%)	⬇️

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