pngmath depth info lost on updates.
When regenerating pages with inline math, but without changing the math, the cached images are used but without "depth" information.
The problem is lines 81-82 of pngmath.py:
if path.isfile(outfn): return relfn, self.builder.env._pngmath_depth.get(shasum, None)
Unless the images have been freshly generated, there is no depth information in self.builder.env._pngmath_depth['shasum']. This information should probably be stored in the _images directory somehow along with the image so it can be used for rebuilding the page with proper baseline offset.