Capture pressure when signing

[UziTech]

update of #503

closes #503
closes #213
closes #379

[szimek]

Thanks! I hope it works ;)

I'm wondering if we could make some of the properties (e.g. _data) public, or rather stop pretending they are private, or provide getters for them, and document them properly. This would allow us to create toWhateverFormat-type function either as a separate function (e.g. toSvg(signaturePad.data), toIsoData(signaturePad.data) that wouldn't have to be loaded if not used, or even as a third-party add-on library.

Also, I just realized that if we stopped supporting Safari 12 and earlier, we could handle pointer events only... However, it might still be too early - I don't know what types of projects this library is used in.

[UziTech]

_data is kind of public with the toData() method. I was thinking about that too. Instead of having all of these to* methods just making them separate packages.

Should I remove the toISOData method and let that be a separate package?

[szimek]

I forgot about toData() 🤦 .

I think it would be a good idea to extract it. I'm just wondering about API - if these toWhatever functions should accept the whole signaturePad instance or just the result of toData(). E.g. toSVG (which theoretically could also be extracted to a separate function, unless we treat it as a core feature) uses some additional properties like dotSize.

[UziTech]

One of my next changes was going to put more of those properties into the point groups so we can save information like max/min width and dot size, etc. per line segment. (PR #571)

I feel like toData() should give all of the properties.