By default, st's window size always snaps to the nearest multiple of the
character size plus a fixed inner border (set with borderpx in
config.h). When the size of st does not perfectly match the space
allocated to it (when using a tiling WM, for example), unsightly gaps
will appear between st and other apps, or between instances of st.
This patch allows st to resize to any pixel size, makes the inner border
size dynamic, and centers the content of the terminal so that the
left/right and top/bottom borders are balanced. With this patch, st on a
tiling WM will always fill the entire space allocated to it.
