Feature request: return match percent from comparePdfToSnapshot

[vaernion]

Currently comparePdfToSnapshot returns true/false and we have to look at the generated heatmap diff to know how much they differed.

If comparePdfToSnapShot could return the equivalent of tolerance [0..1] as well it would be easier to calibrate the appropiate tolerance to use as well as programatically log the difference etc.

It would be a breaking change to make comparePdfToSnapshot return for example { matched: boolean, difference: number}, but maybe worth it?

[moshensky]

Could be done. Return a proportion of different pixels (0-1), where 0 means the two images are pixel identical.

I'm a bit reluctant to break the interface, but I see no major issues of adding a second function with a different result type.