Layout thrashing in data label creation

[dpraul]

Description

When data labels are enabled, chart generation in data-dense charts takes a particularly long time. This is because the label-creation code calls getBoundingClientRect on each point created to ascertain the width/height (depending on chart type). Calling this function in the way it is done causes the browser to perform a synchronous reflow of the entire page - performing it over many points repeatedly does this for each point, which is a huge performance bottleneck called layout thrashing.

Steps to check or reproduce

1. Build a chart that uses data.labels
2. Observe that redrawText takes an inordinate amount of time as a result of the layout thrashing.

The screenshot below captures the performance profile for this function. Observe the red flags all noted below redrawText - this is the profiler noting all the reflows occurring as a result of the call to getBoundingClientRect.

Related to #757