The offset at which stop 0.0 of the gradient is placed.
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).
For example, a begin offset of (-1.0, 0.0) is half way down the left side of the box.
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.
final AlignmentGeometry begin