[Impeller] Implement drawShadow

[bdero]

Doesn't include the ambient occlusion part (which has a comparatively tiny impact on the shadow color).
The accuracy of these shadows will improve a bit more when I land improvements to the rrect approximation.

Skia:

Impeller:

Fixes flutter/flutter#104715

[flutter-dashboard]

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[bdero]

(Still needs a display list test)

[bdero]

I expect a low impact to gallery raster time benchmarks.

[bdero]

Added a display list playground to demonstrate the behavior (and problems with the approximation):