Val viewport unit variants

[ickshonpe]

Objective

Add viewport variants to Val that specify a percentage length based on the size of the window.

Solution

Add the variants Vw, Vh, VMin and VMax to Val.
Add a physical window size parameter to the from_style function and use it to convert the viewport variants to Taffy Points values.

One issue: It isn't responsive to window resizes. So flex_node_system has to do a full update every time the window size changes. Perhaps this can be fixed with support from Taffy.

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Changelog

• Added Val viewport unit variants Vw, Vh, VMin and VMax.
• Modified convert module to support the new Val variants.
• Changed flex_node_system to support the new Val variants.
• Perform full layout update on screen resizing, to propagate the new viewport size to all nodes.

[mockersf]

Fixes DioxusLabs/taffy#239 for me 😄

[ickshonpe]

Having to do a full re-layout on window size changes is annoying, but I don't think even a large FPS drop when resizing really matters that much.

[alice-i-cecile]

Well done and useful. Agreed on the relayout on window size change. I don't see any clean way around that for now, so we can wait and try something more involved if it's a problem.

[ickshonpe]

A hack I've come up with is to have a system that runs on window resize events which iterates through all Style components and marks any with viewport values as changed. I don't think the extra complexity is worth it though for the very minimal and situational performance gains.