Pixel size change was not signalled upon start causing HiDPI displays to
show incorrect rendering. Also fixed the signalling to occur when correct
OpenGL context has been made current.

Change-Id: I69b5e64d3641c81c98880fbe047d71930cc5a16c
Task-number: QTBUG-45692
Reviewed-by: Tomi Korpipää <tomi.korpipaa@theqtcompany.com>
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>