Fixes behavior of drawableSurfaceWidht/Height

drawableSurfaceWidth/Height methods now respect maximum drawable
size and use physical (rendering target) rather than logical (QML layout)
pixel units.

Change-Id: Ice968e6ca541d20eb997eb8f6066df8f38c64666
Task-number: QTBUG-45236
Reviewed-by: Tomi Korpipää <tomi.korpipaa@theqtcompany.com>
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>