Add ComputedNode.logical_size() method

[viridia]

What problem does this solve or what need does it fill?

When dragging, we often need to compute the relative position of the mouse pointer based on the width and height of the widget. However, node.size() returns the physical size, not the logical size of the entity. This means that the node size and the mouse position are in different units if the window scale factor is not 1.

Fortunately, the node contains an inverse_scale_factor property, so getting the logical size is trivial: multiply the physical size by inverse_scale_factor. However, new users may not realize that they need to do this multiplication, especially if they are developing on a window system with a scale factor of 1.

What solution would you like?

Add a logical_size() method to ComputedNode which simply returns the size * inverse_scale_factor.

Additional context

This is a super-trivial addition, and would be a great starter task for a novice user who is looking for a way to get started hacking on Bevy.

[ickshonpe]

We had this and a couple of other logical coords helpers before but I removed them because there were so many recurring bugs due to mixing up the coordinate systems.

I'd rather have something more separated than a logical size method again personally, maybe a new struct you can construct from a GlobalTransform and ComputedNode with support for GlobalTransform rotation and scale as well and lots of geometric helper methods for working with all the different padding and border and content boxes etc.