Bugfix for transformations on Arcs

[kennyweiss]

When loading an SVG with rounded rectangles and transforms, the Path transform function gave the following error when transforming some of the Arc paths:

TypeError: ufunc 'arctan2' not supported for the input types, and the inputs could not be safely coerced to any supported types according to the casting rule ''safe''

After digging into it, some rotations generated eigenvectors with complex values rather than real values.

The fix was to use the Hermitian form (np.linalg.eigh) instead of np.linalg.eig for the eigenvector/value computations which is guaranteed to return reals. As far as I can tell, this is valid based on how the D matrix is generated as invT * Q * T.

The following SVG is (close to) a minimal reproducer:

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<svg width="416.53091" height="506.883"
   viewBox="0 0 416.53091 506.88299"
   version="1.1"
   xmlns="http://www.w3.org/2000/svg" >
  <g transform="translate(-104.31227,-107.15344)" style="display:inline">
    <rect
       x="116.40955"
       y="202.60011"
       width="32"
       height="10"
       rx="5"
       ry="5"
       fill="#8338ec"
       transform="rotate(-8)"
       id="rect10" />
  </g>
</svg>

I added a test to ArcTest that triggers the error before the bugfix and is resolved by the bugfix.

[mathandy]

LGTM, just the one suggestion. Thanks @kennyweiss !