At the moment, only menus and tooltips have a shadow. However, other
elements can benefit from having a shadow. This PR adds shadows to
panels in overlapped regions. Makes the default shadow size slightly
larger, but less opaque so it's not so prominent. While dragging a
panel this makes the shadow larger for a "floating" effect.
Pull Request: https://projects.blender.org/blender/blender/pulls/139847
0a22fbc153