This fixes a couple of subtle rectangle select behaviors:
* Corrects how selection rolls over when crossing the x-boundary; this
was mentioned in #1021, this properly corrects it so both sides of the
x-boundary do not share characters.
* Corrects a minor quirk in the selection of initial cells in a
selection - this can be more readily observed when selecting a single
line with rectangle select. To correct this, we only use the x axis
when calculating this instead of both x and y.
aeaf5ea467