De-flake unit tests failing due to WebGL context loss

[jimbojw]

Browsers can only support a limited numbers of concurrent WebGL contexts. When the limit is exceeded, older contexts are dropped. This can cause errors and test failures.

Megaplot is designed to be used for big visualizations that take up a lot of screen space. In a production environment, exceeding the context limit is less likely. But the karma unit tests create many contexts to exercise various features and are starting to become flakey.

Task: De-flake the karma unit tests so that they succeed even if contexts are lost. This may require somehow forcing tests to execute sequentially. Karma appears to somehow interleave some of the code under test.

This effort may be aided by implementing an explicit Scene destroy() method as described in #47.