Fix -Wcast-qual errors in QtCanvas3D

There's no need to cast the logging context to call the const function
isDebugEnabled().

Change-Id: Iee8cbc07c4434ce9b560ffff13cd99f702512bc7
Reviewed-by: Tomi Korpipää <tomi.korpipaa@theqtcompany.com>
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>