(QNX) Make it possible to set physical screen size

(QNX) Make it possible to set physical screen size

It can happen that the system is reporting a screen
width or height of zero, if the system does not know the
size instead of returning an error. You can either
set a define QQNX_PHYSICAL_SCREEN_WIDTH/QQNX_PHYSICAL_SCREEN_HEIGHT
or set it as environment variable QQNX_PHYSICAL_SCREEN_SIZE.

The preference order is:
1) Take detected size
2) Take size from env var
3) Take defined size if any define is set

Change-Id: I493a2c8988a225224587a6c323efeb9e9b7b5a54
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>