The hypothesis behind this would be: The rendering apparently leads to
increasing glyph matrix sizes; maybe it does something with fonts, no
idea. The memset clears all glyphs which I can only imagine to play a
role when we somewhere in the display code compare desired and current
glyphs, and that fails with the memset and didn't fail without it. Where
that might be I have no idea.

Anyway, with the loop in my diff it should be as before, and
glyph::frame is unused in the window-system case, so...