Add first-class support for blurring what's underneath a Rectangle

[tronical]

Similar to drop-shadow-* properties we should have blur-background-* properties (or similarly named), to apply a blur effect to what's underneath.

This is not quite a sub-tree effect but should apply to whatever was rendered "before". This is expensive in the implementation (so needs to be used sparingly), but in that shape it would be super convenient for designers (this issue is based on that feedback).

Connected conceptually to #612.

[chianti-ga]

any news about this?

[tronical]

Unfortunately no news yet.

[flukejones]

We would like to have this at JTE for our front panel.

[vgarleanu]

What would it take to implement this feature? What sort of difficulty?

[tronical]

If we say the this property applies to only what’s rendered exactly underneath the rectangle it’s used on, then I think this could be implemented by means of rendering the tree of items again starting at the root but (1) clipped to the rectangle and (2) not descending into the rectangle itself. This second pass would render into a layer and the blur would have to be applied there.

But I think this may the wrong approach.

Mayhe the existence of a blur in the tree means that we need to separate the scene into multiple layers (think multiple blurs) and the compose the layers accordingly. This avoids a double pass of the tree at the expense of more texture memory.