LinearGradients have an implicit start point at 0,0 and an end point at 256,0. The transform
parameter can be used to adjust them to the desired location.
RadialGradients have an implicit center at 0,0 and a radius of 128.
The helper functions: new_linear_gradient, new_radial_gradient and new_two_circle_radial_gradient
allow the gradients to be constructed with easier to understand inputs.
The transform parameter maps user space to source space. This means that setting the same transform
on the draw target as the source will have the effect of canceling out.