Allow multiple varying height rows to be allocated to the char atlas at once

[Tyriar]

Since underlines are now rendered in the atlas we can get into this situation:

While it is pretty unlikely for this to happen in practice, this is a change I'm wanted to do for a while. Basically allow multiple active rows to exist in the char atlas at once, in practice this should typically put all the short chars ~, -, _, etc. into that row and save a bunch of vertical space in the texture.