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>