Fixed signaling of pixel size change.

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>