Markers drawn in the stack chart are not very visible when they're small

Here is an example: https://share.firefox.dev/3x6ebx8 from https://phabricator.services.mozilla.com/D212717

I think the issue is that this was implemented by making the markers drawn by the stack chart implementation.
Instead I think we should use the marker chart implementation in its own separate canvas. The nice consequences would be that the marker design would be the same, and we'd get the context menus and tooltips (see also #2354, #2355) for free.

I don't know how hard or easy this would be though. It would need some additional investigation to estimate the complexity.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-802)