SizeRequest: Round px values up for min CSS sizes

SizeRequest: Round px values up for min CSS sizes

Otherwise, requesting a min size in em where the equivalent in px had a
fractional part would lead to the widget getting allocated 1 too few px.
You could see this in the CSS property vs. allocation in the Inspector.

Note that margin/border/padding are left alone: the rationale is that we
do as browsers do, and Benjamin said we already do that for those,
whereas his tests on min-(width|height) showed otherwise. My subsequent
analysis indicated it to be far less clear-cut than that, but he remains
unconvinced that we should ceil() all the things! So just do these ones.

https://gitlab.gnome.org/GNOME/gtk/issues/1088