[out] Pointer to a
RectF
object that receives the rectangle that defines the boundaries of the
gradient. For example, if a linear gradient brush is constructed with a
starting point at (20.2, 50.8) and an ending point at (60.5, 110.0),
then the defining rectangle has its upper-left point at (20.2, 50.8), a
width of 40.3, and a height of 59.2.