Fixing the canvas resize to behave more like in HTML world.

Separating the pixel size from the layout size like in HTML makes Canvas3D
behave more like the WebGL canvas and reduces delta when porting code.

Change-Id: Icb9e083839a42d0cd5c3823fa37c2e2f2a913e74
Reviewed-by: Tomi Korpipää <tomi.korpipaa@digia.com>
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>