FIX: Annotation labels are packed RGB, not RGBA

[effigies]

Contrary to FreeSurfer documentation, FreeSurfer code uses only the RGB values of colors to identify to annotation labels, ignoring the transparency value.

It is worth noting that FreeSurfer stores a transparency value, which is 255 - alpha. I'm somewhat inclined to continue returning raw transparency values, as we are not returning labeled data structures, but if people feel strongly that we should return alpha values, I'm okay with that, as well.

Fixes #649.

[coveralls]

Coverage remained the same at 91.826% when pulling 7b59675 on effigies:fix/annot_labels into be35aca on nipy:master.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@be35aca). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #666   +/-   ##
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  Coverage          ?   88.86%           
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  Files             ?       93           
  Lines             ?    11378           
  Branches          ?     1869           
=========================================
  Hits              ?    10111           
  Misses            ?      930           
  Partials          ?      337

Impacted Files	Coverage Δ
nibabel/freesurfer/io.py	94.27% <100%> (ø)

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[effigies]

@pauldmccarthy Are you comfortable with these changes?

[effigies]

Going by the response on the mailing list, Doug at least agrees with this interpretation.

@larsoner says it works for them over in PySurfer in #649 (comment).

Barring objection, plan to merge this fix on Monday.

[effigies]

freesurfer/freesurfer#541 has been merged, so let's get on the same page.

[pauldmccarthy]

@effigies sorry, was away from laptop all of last week - all looks good to me, thanks again.