a1267bc fixed repeated re-appending of '@2x' by moving the modified URL
to a local variable, but it still set hasHiDPI to false permanently;
this meant that icons displayed repeatedly in the same window, like the
tab close button, would only use their 2x versions the first time they
were displayed (in the case of tabs, this is the library tab's invisible
close button, so *no* tabs got visible 2x buttons).
This commit moves the HiDPI check and URL modification out of the render
function and instead runs it a single time when i() is first called.
e8b574da0b