center property
The center of this RadialSteps as an offset into the
(-1.0, -1.0) x (1.0, 1.0) square describing the Gradient
which will be mapped onto the paint box.
For example, an alignment of (0.0, 0.0) will place
this RadialSteps in the center of the box.
If this is an Alignment, then it is expressed as a vector from
coordinate (0.0, 0.0), in a coordinate space that maps the center
of the paint box at (0.0, 0.0) and the bottom right at (1.0, 1.0).
It can also be an AlignmentDirectional, where the start is the left
in left-to-right contexts and the right in right-to-left contexts.
If a text-direction-dependent value is provided here,
then the createShader method will need to be given a TextDirection.
Copied from RadialGradient.
Implementation
final AlignmentGeometry center;