Fix XRImage.colorize to mask integer data with _FillValue

[djhoese]

• Closes Active fire confidence geotiff images do not have transparent background ssec/polar2grid#545 (remove if there is no corresponding issue, which should only be the case for minor changes)
• Tests added (for all bug fixes or enhancements)
• Tests passed (for all non-documentation changes)
• Passes git diff origin/master **/*py | flake8 --diff (remove if you did not edit any Python files)
• Fully documented (remove if this change should not be visible to users, e.g., if it is an internal clean-up, or if this is part of a larger project that will be documented later)

[codecov]

Codecov Report

Merging #109 (3641038) into main (a2ca805) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #109      +/-   ##
==========================================
+ Coverage   92.89%   92.93%   +0.03%     
==========================================
  Files          11       10       -1     
  Lines        3478     3495      +17     
==========================================
+ Hits         3231     3248      +17     
  Misses        247      247              

Flag	Coverage Δ
unittests	92.93% <100.00%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
trollimage/tests/test_image.py	100.00% <100.00%> (ø)
trollimage/xrimage.py	89.23% <100.00%> (+0.10%)	⬆️
trollimage/__init__.py

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[pnuu]

One debug print, otherwise LGTM.

[mraspaud]

Let me check if this works with the existing cloud top height composite...

[mraspaud]

LGTM