Background not redrawn if geometry doesn't change

[ehhov]

If some input line and the next one have exactly the same width and height, the contents of the latter are printed over the previous line making the text overlay the existing one.

Steps to reproduce:

1. Choose a font like Noto Sans, DejaVu Sans, Times, Iosevka, Terminus, etc. and not like Lobster,
2. Make umess print "oe",
3. Make umess print "eo" right after that within the same run.

[kompoth]

That's definitely a bug. I guess I need to understand first my own code, it's been a long time.