feat: add LinesGL Mark

[maartenbreddels]

Does not have feature parity yet with bqplot.Lines. Supports

• x and y
• colors (only the first item is taken, so only a single color)
• opacity (only the first item is taken, so only a single opacity)

To be feature complete for glue-jupyter, we also need line width.

[codecov[bot]]

Codecov Report

Merging #58 (327103b) into master (bcde02d) will increase coverage by 5.38%. The diff coverage is 100.00%.

:exclamation: Current head 327103b differs from pull request most recent head 6b7e58e. Consider uploading reports for the commit 6b7e58e to get more accurate results

@@            Coverage Diff             @@
##           master      #58      +/-   ##
==========================================
+ Coverage   50.00%   55.38%   +5.38%     
==========================================
  Files           6        7       +1     
  Lines         116      130      +14     
==========================================
+ Hits           58       72      +14     
  Misses         58       58              

Impacted Files	Coverage Δ
bqplot_image_gl/__init__.py	40.00% <100.00%> (+4.28%)	:arrow_up:
bqplot_image_gl/linesgl.py	100.00% <100.00%> (ø)

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[maartenbreddels]

This closes https://github.com/spacetelescope/jdaviz/issues/461 To compare, this 300k line segment runs about 0.1 fps using svg (bqplot.Lines) and using LinesGL runs quite smooth at ~12 fps under the profiler in chrome (the gif actually doesn't do it justice):

[maartenbreddels]

@astrofrog I'm gonna merge this so this can be tested with https://github.com/glue-viz/glue-jupyter/pull/227 . This only adds a new feature, so totally backwards compatible.