blendMode property
The BlendMode utilized in painting this Foil's gradient
over its child.
Default is BlendMode.srcATop, which will envelop the child in the gradient entirely, masking the gradient out in areas where the child is transparent.
Note that if the gradient has transparency itself, or if opacity is low enough, then the child will be visible through it. Foils.oilslick is one such
Gradientwith transparency, for example.
Utilizing BlendMode.srcIn will paint only this Foil's gradient,
masking out the child entirely.
Other BlendModes may be experimented with to achieve neat results
when combined with bespoke Gradients.
Implementation
final BlendMode blendMode;