Adjust menu shadows

[fleabitdev]

This commit makes menu and tooltip shadows slightly fainter and smaller.

It also introduces the option for shadows' top edges to be slightly inset relative to their parent window. This causes menus and tooltips to cast less of a shadow over the widget which opened them.

Closes https://github.com/emilk/egui/issues/3046.

[fleabitdev]

Before/after comparison (animated):

[emilk]

bump