Visible jaggies on some gradient types

[micycle1]

Until now PeasyGradients rendering has been heavily optimised for speed.
However some gradients exhibit numerical artifacts, particularly visible on what are meant to be straight edges.

Fix any gradients with perceptible problems (which may mean giving up some performance in favour of visuals).

Example

The straight lines on this polygonal gradient having 4 sides have visible jaggies.

[micycle1]

Follow up for polygonal gradients

Jaggies fixed using a larger LUT and more precise atan2 approximation.