Correctly handle transforms in SVG arcTo

[ianthomas23]

Fixes #12699.

Rects with rounded corners are currently rendered incorrectly using the SVG output backend. This is due to a bug in SVGRenderingContext2D.arcTo which is called once for each rounded corner. The calculations in arcTo make use of the last rendered point which is in the member variable __currentPosition and is updated at the beginning and end of every SVG path rendered. The bug is that __currentPosition is stored after being transformed by the current transform stack (SVGRenderingContext2D._transform) whereas the arguments to arcTo are not transformed. Combining transformed and untransformed points can clearly give incorrect results. Up until recently this has not been a problem, but now we are using rects with rounded corners that may be rotated using the angle property or translated if they are part of a title.

The fix is to inverse transform the __currentPosition before using it in arcTo. I have added an inverse() function to AffineTransform that is implemented using DOMMatrix.inverse so that we don't have to implement our own maths. There are a number of existing visual tests that show this working correctly now. Note that even the unrotated visual image is altered as it now correctly has a half pixel offset that would not have been visible to the typical human eye. I've also added a unit test for AffineTransform.inverse.

Simple demonstration code:

from bokeh.layouts import row
from bokeh.plotting import figure, show

ps = []
for backend in ('canvas', 'svg'):
    p = figure(width=500, height=250, output_backend=backend, title=backend)
    ps.append(p)
    p.rect(x=[0, 1, 2, 3], y=0, width=0.8, height=0.4, angle=[0, 0.3, 0.6, 0.9],
           line_width=5, fill_color="orange", line_color="black", fill_alpha=0.8,
           border_radius=35)

show(row(ps))

which gives before this PR:

and after:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5854a25) 92.56% compared to head (f252952) 92.56%.

Additional details and impacted files

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##           branch-3.4   #13640   +/-   ##
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  Coverage       92.56%   92.56%           
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  Files             323      323           
  Lines           20509    20509           
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  Hits            18985    18985           
  Misses           1524     1524