Remove device pixel ratio scaling from PixmapRequest

The PixmapRequest constructor expected width and height in logical
pixels, and scaled them to device pixels.  However, not knowing what
screen the request is for, it had to use qApp->devicePixelRatio()
for the scaling.  That value may not be correct if not all screens
use the same scaling.

Fix this by introducing a new constructor that takes the device
pixel ratio as an additional argument, and deprecating the old
constructor.