Semi-transparent-cover doesn't cover everything and renders wrongly

[JoshuaLampert]

You can find an MWE at : https://typst.app/project/rO30herYanvASbIkJdjjW2.
This issue has two (maybe related?) sub-issues:

1. In the first subslide we see that the lower line of the equation is not central, the "x" is not in math font, and the vector is smaller than in should be.
2. Not everything is covered. Only a strip, which is not high enough covers the central part of the equation, but the upper and lower parts are still visible without transparency.

[OrangeX4]

This is due to the fact that currently semi-transparent cover is very hacky and may be difficult to fix. Maybe we can consider changing the alpha channel of text color instead of using a semi-rect.