Preserve known dimensions in empty inline layout short-circuit

[nicoburns]

Summary

Fixes the css/css-flexbox/flexible-box-float.html regression on #625.

When an inline formatting context contains only out-of-flow content (all children absolutely positioned or floated — e.g. a <p>&nbsp;</p> flex item whose only text is whitespace and whose box is floated), compute_inline_layout short-circuits and returned Size::ZERO, discarding the definite known_dimensions taffy passed for the item (its flexed target size, e.g. 300x30). The flex items therefore collapsed to 0x0 and the test's red fail-flag became visible.

 return LayoutOutput::from_outer_size(
-    Size::ZERO.maybe_max(container_pb.sum_axes().map(Some)),
+    Size {
+        width: known_dimensions.width.unwrap_or(0.0),
+        height: known_dimensions.height.unwrap_or(0.0),
+    }
+    .maybe_max(container_pb.sum_axes().map(Some)),
 );

Together with DioxusLabs/taffy#1081 (which fixes flexbox-min-height-auto-002b.html), both flexbox regressions on #625 pass. Verified with the WPT runner and cargo fmt/clippy.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/40e9a29868044e77a6dbf77a7b4b11e3
Requested by: @nicoburns

WPT results

8 newly passing, 2 newly failing (net +6).

Full diff (10 changed tests)

+ Fail => Pass css/CSS2/linebox/line-height-applies-to-016.xht
- Pass => Fail css/CSS2/normal-flow/inline-block-zorder-002.xht
- Pass => Fail css/CSS2/normal-flow/inline-table-zorder-001.xht
+ Fail => Pass css/CSS2/normal-flow/max-height-applies-to-016.xht
+ Fail => Pass css/CSS2/normal-flow/max-width-applies-to-016.xht
+ Fail => Pass css/CSS2/normal-flow/min-width-applies-to-016.xht
+ Fail => Pass css/CSS2/text/white-space-007.xht
+ Fail => Pass css/css-animations/animation-name-in-shadow-part-inner-match.html
+ Fail => Pass css/css-flexbox/flexible-box-float.html
+ Fail => Pass css/css-values/lh-unit-002.html

_{Generated by the WPT workflow.}

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