Improve "x" for muted strings

[bkeepers]

Manually drawing the "x" for muted strings (instead of using text rendering) results in better size and style matching with "o" for open strings. Notice the differece in line height and weight between the "x" and "o" in the diagrams below.

Before

After

[!NOTE]
I know this library has been stagnant for a while, so I'm not sure if my recent PRs will be merged or not. In the meantime, I'll be maintaining a fork with all my improvements at chordbook/vexchords.