FIX: Fixes for OpenCL and new example

@rossant it looks like you might have merged some `pyopencl` commits into `master` that broke Travis (and also made tests fail on my system):

https://github.com/vispy/vispy/commit/6dcb94900ead6a56b9b336209f23a7f773e60de9

See last `master` build result:

https://travis-ci.org/vispy/vispy/builds/19449966

This PR should fix that problem.

This PR also fixes some PEP8 + pyflakes violations introduced in a subsequent merge that looks like it might have been by @rougier:

https://github.com/vispy/vispy/commit/7dd8fb832a5fbf38ea6ebc48977929ebaf8c8b50

Is there a policy for checking the Travis state before merge? Merging via PR as opposed to direct push has an advantage in this regard, although it's less convenient in some circumstances. It's also possible these merges were expedited as part of the code camp so they could be used quickly, which would make sense for that situation.