The :matrix property is a #GVariant property to specify where and what kind of white spaces to draw.
The #GVariant is of type "au", an array of unsigned integers. Each
integer is a combination of #GtkSourceSpaceTypeFlags. There is one
integer for each #GtkSourceSpaceLocationFlags, in the same order as
they are defined in the enum (%GTK_SOURCE_SPACE_LOCATION_NONE and
%GTK_SOURCE_SPACE_LOCATION_ALL are not taken into account).
If the array is shorter than the number of locations, then the value for the missing locations will be %GTK_SOURCE_SPACE_TYPE_NONE.
By default, %GTK_SOURCE_SPACE_TYPE_ALL is set for all locations.
Whether the #GtkSourceSpaceDrawer:matrix property is enabled.