Initial, experimental WebGL rendering support

[maxkfranz]

Description of new feature

What should the new feature do? For visual features, include an image/mockup of the expected output.

Add support for experimental rendering using WebGL. Not all rendering features will be initially supported with WebGL, and improved WebGL support will be added over time.

Note that until the WebGL rendering feature has been marked as final, the API and behaviour of the renderer will be subject to change.

This initial version in 3.31.0 will be enabled via options.webgl: true. It will support:

• Node styles
• Labels
• Straight edges
• Bezier edges
• Triangle arrows only

If you notice bugs or missing features that are important to you, file an issue on Github.

See also:

• Performance enhancements using the GPU #3261

Motivation for new feature

Describe your use case for this new feature.

• Improved rendering performance
• Getting community feedback for areas of improvement

For reviewers

Reviewers should ensure that the following tasks are carried out for incorporated issues:

• Ensure that the reporter has adequately described their idea. If not, elicit more information about the use case. You should iteratively build a spec together.
• Ensure that the issue is a good fit for the core library. Some things are best done in extensions (e.g. UI-related features that aren't style-related). Some things are best done by app authors themselves -- instead of in Cytoscape libraries.
• The issue has been associated with a corresponding milestone.
• The commits have been incorporated into the unstable branch via pull request. The corresponding pull request is cross-referenced.

[mikekucera]

Work is happening in the feature/gpu branch.