Rendering performance improvements

[abought]

Purpose

LocusZoom becomes very slow when loading dense regions (with a lot of variants). Some users would like to increase max_region_size and/or render modern datasets that may be imputed to better panels.

This ticket contains some initial low-hanging performance improvements:

• Use fewer system resources/ listeners/ function calls to render each scatter plot point (cache computed values, event bubbling, etc)
• Provide an option to coalesce adjacent points in the scatter plot. This can save reduce the number of points to render by 40-50% for a narrow region or noisy plot, and >90% for a GWAS with a few strong hits in a wide viewing region.

Summary of changes

• The new scatter plot directives coalesce._ control whether to group together nearby points, and if so, what parameters to use. By default it groups together adjacent points within ~1 circle-diameter distance of each other in x and y.
  • Coalescing is turned on by default for association plots (only). You may wish to adjust this behavior and options if you use a different y-axis range for your plots (eg, user-selectable "toggle y-axis display" behaviors)
• Remove shift-click "multiselect" from plots: few people seem to use this feature

Bugfixes:

• Correctly remove old scatter plot labels on toggle/re-render
• Fix double-escaping issue with label display values

TODO

• Cache the most frequently re-computed values
• Explore refactoring event listeners to use bubbling- do we really need 1000 events for 1000 scatter plot points?
• Deduplicate style declarations when rendering
• Explore coalescing nearby points