Implement math-mode text for Windows

matplotlib uses 2 strategies for rendering text: a simple text render, and an outline-based render that is used when the text contains math/TeX expressions.

The Windows canvas backend currently can't render those math expressions. As a workaround, toga-chart does a naive stripping of math expressions, and renders them with the simple text renderer.

The fix for this will almost certainly be on the Toga canvas side; However, the workaround in toga-chart will need to be removed to test any potential fix (and, once it *is* fixed, be removed permanently).

Start by tracing the math-expression stripping workaround in toga-chart and then inspect the Toga Windows canvas path responsible for outline-based text rendering. Done means math/TeX expressions render correctly on Windows without the workaround; verify with a chart containing math text.

Written by the indexing model from the issue text.