Fix
'friend' keyword needs 'class'
error introduced by 035c29e6a2 (Make wxRescaleCoord() safer by requiring
explicitly using From/To, 2021-07-13) which broke the build with gcc
4.4.7 used under CentOS 6.
Closes https://github.com/wxWidgets/wxWidgets/pull/2450
Use the same scaling function as elsewhere instead of using
ceil/floor().
Provide wxRescaleCoordWithFrom specialization for int to allow using it
with just single coordinates too.
They're probably not that useful in application code, which should just
use wxDPIChangedEvent::Scale() instead, so move them to a private
header instead of making them part of the public API.
