Webgl: Handle context loss when there are too many contexts

[Tyriar]

Webgl addon added in #1790

See Tyriar#5 and the associated comments

[Tyriar]

This is getting a fix in #3276 from @meganrogge where embedders will be able to easily handle context loss however they want, a simple handling of it is to dispose the addon and fallback for example.

We could consider further improvements but we'll close this issue off with that.