Fix image widening on xxhdpi+ devices
Our existing CSS only widens images to 60% for xxhdpi+ devices such
as the Samsung Galaxy S4, because min-device-width is dependent on
screen density; hence, for the S4, (min-device-width: 768px) was
evaluating as true.
So I've added a rule to make the width and max-width 100% after all
for devices in portrait orientation with min-device-width 768px where
the device pixel ratio >= 3.