On iPhone 6(s) Plus devices, or when display zoom is enabled in an iPhone 6,
the render buffer is scaled before being output on the physical display.
We have to take this into account when computing the physical size.

Task-number: QTBUG-50941
Change-Id: I318f3a866d039fccf0ba08f381fc9d8bcd676acd
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@theqtcompany.com>