qt: add QT_COORD_TYPE to double option

On ARM-based platforms, qreal is a typedef for float for performance
reasons. But we may want a double precision anyway.

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>