layout: Implement support for line-height and vertical-align (#30902)

* layout: Implement support for `line-height` and `vertical-align`

This is an initial implementation of proper `line-height` and
`vertical-align` support. While this change includes the bulk of the
work there are still many missing pieces for full support. In particular
some big missing things are:

 - Flex containers do not properly compute their baselines. The idea is
   to tackle this in a followup change. This causes various flex tests
   to start failing because everything used to be top aligned.
 - The implementation of the line-height quirks (only active in quirks
   mode) are incomplete. While the quirk works in many cases, there are
   still some cases where it is handled incorrectly. This requires more
   redesign and refinement, better suited for a followup.
 - Most of the features are CSS 3 such as precision control of the
   baseline and first and last baselines are not implemented. This
   change gets us close to CSS 2.x support.

While there are many new test passes with this change some tests are
starting to fail. An accounting of new failures:

Tests failing also in Layout 2013:
 - /css/css2/positioning/toogle-abspos-on-relpos-inline-child.html (only passes in Chrome)
 - /css/CSS2/fonts/font-applies-to-001.xht (potentially an issue with font size)

Invalid tests:
 - /css/CSS2/visudet/inline-block-baseline-003.xht
 - /css/CSS2/visudet/inline-block-baseline-004.xht
 - These are are failing in all browsers. See https://bugs.chromium.org/p/chromium/issues/detail?id=1222151.

Missing table support:
 - /_mozilla/mozilla/table_valign_middle.html

Missing `font-size-adjust` support :
 - /css/css-fonts/font-size-adjust-zero-2.html (also failing in 2013)

Incomplete form field support :
- /html/rendering/widgets/the-select-element/option-add-label-quirks.html (label isn't rendered so button isn't the right size in quirks mode due to line height quirk)

Need support for calculating flexbox baseline:
 - /css/css-flexbox/fieldset-baseline-alignment.html
 - /css/css-flexbox/flex-inline.html
 - /css/css-flexbox/flexbox-baseline-multi-line-horiz-001.html
 - /css/css-flexbox/flexbox-baseline-single-item-001a.html
 - /css/css-flexbox/flexbox-baseline-single-item-001b.html

Failing because we don't create anonymous inline boxes for text children of blocks:
- /css/CSS2/linebox/anonymous-inline-inherit-001.html

Passes locally (potentially related to fonts):
 - /css/CSS2/css1/c414-flt-fit-004.xht
 - /css/css-transforms/transform-input-017.html
 - /html/obsolete/requirements-for-implementations/the-marquee-element-0/marquee-min-intrinsic-size.html
 - /css/css-fonts/first-available-font-005.html
 - /css/css-fonts/first-available-font-006.html

* Some cleanups after live review with @mukilan

Also update results.

components/gfx/font.rs

@@ -121,6 +121,27 @@ pub struct FontMetrics {
     pub line_gap: Au,
 }
 
+impl FontMetrics {
+    /// Create an empty [`FontMetrics`] mainly to be used in situations where
+    /// no font can be found.
+    pub fn empty() -> Self {
+        Self {
+            underline_size: Au(0),
+            underline_offset: Au(0),
+            strikeout_size: Au(0),
+            strikeout_offset: Au(0),
+            leading: Au(0),
+            x_height: Au(0),
+            em_size: Au(0),
+            ascent: Au(0),
+            descent: Au(0),
+            max_advance: Au(0),
+            average_advance: Au(0),
+            line_gap: Au(0),
+        }
+    }
+}
+
 /// `FontDescriptor` describes the parameters of a `Font`. It represents rendering a given font
 /// template at a particular size, with a particular font-variant-caps applied, etc. This contrasts
 /// with `FontTemplateDescriptor` in that the latter represents only the parameters inherent in the