Gating: ensure no bugs in WebGL updates

[thejohnhoffer]

todo list for manual testing in Chrome:

• Ensure /download_channels_csv matches release
• Ensure /download_gating_csv matches release
  • For *_channel_ranges.csv files
  • For *_cell_encodings.csvfiles
• Ensure /upload_channels matches release
• Ensure /get_uploaded_gating_csv_values matches release
• Ensure /save_channel_list matches release
• Ensure /save_gating_list matches release
• Ensure /get_saved_gating_list matches release
• Ensure /get_saved_channel_list matches release
• Ensure "auto" GMM works:
  • for rgba channels
  • for gated markers
• Ensure up to 4 channels supported
• Stress test by rapid toggling of:
  • rgba channels on/off, sliders
  • gated channels on/off, sliders
• Ensure numerical min/max inputs work
• Ensure color selection works
• Ensure or-mode pie charts work
• Ensure outline mode works^1
• Ensure OpenSeadragon icons work^2
• Ensure right-click picking works

^1 Follow up with @nchuynh on issue with non-adjacent cells (Issue #100)
^2 Full-screen Icon works but logs error to console (PR #103)