When starting a scroll while the mouse is over a mermaid diagram, the
scroll sometimes propagates to the iframe, preventing the parent page
from scrolling. Fix this by disabling scroll inside the iframe. This is
not a problem because those frames are never meant to scroll. Bug seems
to affect Firefox only.
![scroll](https://user-images.githubusercontent.com/115237/203847578-6831e3c8-9df4-4577-8501-822fb9ea1278.gif)