Add support for an inner shadow to MultiEffect

We need an inner shadow to replicate macOS's native Switch. This isn't available in MultiEffect, and would be nice to have.

I tried copying InnerShadow:

i tried it out but i'm not sure if it's worth it: https://codereview.qt-project.org/c/qt/qtdeclarative/+/495640/4..5

there are a lot of extra files there, and i have to use a non-zero verticalOffset otherwise it's too heavy in the corners (i went for the FastInnerShadow implementation rather than the GaussianBlur one), but a verticalOffset of e.g. 1 doesn't look quite right

If the implementation of InnerShadow is used, we should check if any of the existing bug reports are still valid too: https://bugreports.qt.io/issues/?jql=text%20~%20innershadow%20ORDER%20BY%20created%20DESC