Previously, we were taking thresholds and min/max sizes into
account when choosing the best theme dir, but when it came
to loading the icon, we always scaled icons from scalable
directories all the way, ignoring the min/max size limits.

This commit changes things around so that we now load icons
in Threshold directories at their nominal size, and scale
icons in Scalable directories only up to the specified limits.

To override this and keep the previous behaviour of scaling
all the way to the desired size, use the GTK_ICON_THEME_FORCE_SIZE
flag.