…buffer

This allows to cleanup resources earlier if they stop being used. Right
now all resources were cleaned up when the context was destroyed, this is
a slightly better approach.

We ignore the possible failure of the send() call, since we don't keep
track of these resources from the `WebGLRenderingContext` structure, so
a texture could be destroyed after the context and give us problems.

I can't reproduce locally the failure that produced them, and since the
builds that triggered them are deleted for some reason (like
http://build.servo.org/builders/linux-dev/builds/419), it could be a
good moment to troubleshoot it.

If merged closes #7931

This allows running WebGL reftests in wpt.

This new version shold prevent crashing on the linux buildbots.