Support use of Qt::AA_EnableHighDpiScaling and 
QT_AUTO_SCREEN_SCALE_FACTOR.

This makes the high-dpi scaling system use the screen
configuration set with QT_QPA_EGLFS_PHYSICAL_WIDTH and
QT_QPA_EGLFS_PHYSICAL_HEIGHT.

Implement QEglFSScreen::pixelDensity() and make it return
the scale factor for the display. The scale factor is computed
as the ratio of the logical DPI to the base DPI.

Change-Id: I14ed5da058024128479cb5508e056c39bd2f7563
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>