Plane matrix test

This is an extension to the webtest/render_performance/ page to try and improve speed of retrieving all planes of an image (since this is useful for 'whole stack' rendering - E.g. http://codepen.io/will-moore/pen/Beuyc)

To test, choose a multi-plane image in web, note the ID and go to: ```<server>/webtest/render_performance/image/<image-Id>/```

Load all planes as single planes by clicking "Load All". Note the duration to load all planes.

Now look below and click "Load Plane Matrix" and note the time taken to load all planes as a single 'matrix'. Also experiment with splitting the matrix into several portions using the select input. This may decrease the duration to load all planes since sub-portions are loaded simultaneously (up to 5, since we have 5 workers)?

--no-rebase