Store simulated pressure

[adil192]

Motivation

It would be helpful to be able to store the simulated pressure values in order for my project to be able to compress lists of points.

There will often be multiple points very close together which is used for the simulated pressure calculation in perfect_freehand but they're a waste of storage when written to a file.

I also believe I might be able to speed up canvas rendering by reducing the number of points.

(Also the reason that I store the original points and not the resulting stroke is that I want to give users the ability to change strokes' color and thickness retrospectively.)

Proposed usage

final points = [Point(0,0), ...];
final stroke = getStroke(
  points,
  simulatePressure: true,
  // this flag writes the new pressures to the given list of points
  rememberSimulatedPressure: true,
);
print(points.first.p); // e.g. 0.82615167

Alternatives

perfect_freehand could offer to compress a list of points like I mentioned before, e.g.

final points = [Point(0,0), ...];
final compressedPoints = approximateStrokePoints(
  points,
  simulatePressure: true,
  maxDeviation: _someConstant,
);