Optimize WebGL memory usage

[mikekucera]

The WebGL mode creates textures at runtime for node bodies and labels. This allows all existing node and label styles to work with WebGL.

For some networks the textures are taking up an unreasonable amount of video memory, causing crashes.

It seems the best way to fix this issue is to compress the textures using one of the supported WebGL texture compression formats. Since we are not loading static assets, we are creating textures at runtime, that means the compression must also happen at runtime.

Further discussion on this topic, and any other WebGL performance issues, is here: #3325

For reviewers

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

• Ensure that the reporter has included a reproducible demo. They can easily fork this JSBin demo: http://jsbin.com/fiqugiq
• The issue has been associated with a corresponding milestone.
• The commits have been incorporated into the corresponding branches. Bug-fix patches go on
  • master,
  • unstable, and
  • the previous feature release branch (e.g. 1.1.x if the current release is 1.2).
• The issue has been labelled as a bug, if necessary.