getBlend<S, T>  abstract method 
- PixelFormat<
S, void> srcFormat, - PixelFormat<
T, void> dstFormat 
Returns a function that blends two pixels together.
Implementation
T Function(S src, T dst) getBlend<S, T>(
  PixelFormat<S, void> srcFormat,
  PixelFormat<T, void> dstFormat,
);