WISH: set group opacity in Draw (similarly to SVG)

[hiiamboris]

Will be useful for some animated transitions, e.g. (dis)appearance of whole UI layout. Imagine two layouts: layout1 and layout2 and their opacities tied as opacity1(t) + opacity2(t) = 1.

Currently it's only possible by rendering this layout on an image and modifying opacity of all pixels of that image, which is rather heavy.

Need to check if Draw backends have support for this, but since it's a core SVG feature the bet is on.