client/webgl: Split off sub-templates webglControls, webglCanvas
webglControls: Remove references to slider values from the template

Re-rendering the slider on each change will cause the slider change
handler to be also re-run and eventually we will get a storm of events
on each change. We must be much more cautious about the Reactivity
feature of Meteor when associated with interactive elements, overally.
webglControls: gamma_current is not 10^gamma but just gamma
webgl: Tidy up formatting etc.