Center-align all text vertically (#5117)

* Closes https://github.com/emilk/egui/issues/4929
* Builds on top of https://github.com/emilk/egui/pull/2724 by @lictex
(ptal!)
* Implement `Center` and `Max` vertical text alignment properly
* Change default vertical alignment of text to centering

The end result is that text centers better in buttons and other places,
especially when mixing in emojis.
Before, mixing text of different heights (e.g. emojis and latin text) in
a label or button would cause the text to jump vertically.

## Before
This is `master`, with custom `FontTweak` to move fonts up and down:
<img width="1714" alt="image"
src="https://github.com/user-attachments/assets/a10e2927-e824-4580-baea-124c0b38a527">
<img width="102" alt="image"
src="https://github.com/user-attachments/assets/cd41f415-197b-42cd-9558-d46d63c21dcb">


## After
This PR, with the default (zero) `FontTweak`

<img width="102" alt="image"
src="https://github.com/user-attachments/assets/15e7d896-66b1-4996-ab58-dd1850b19a63">

<img width="1714" alt="image"
src="https://github.com/user-attachments/assets/54ec708c-7698-4754-b1fc-fea0fd240ec9">

crates/epaint/src/text/font.rs

@@ -468,6 +468,14 @@ impl Font {
         (Some(font_impl), glyph_info)
     }
 
+    pub(crate) fn ascent(&self) -> f32 {
+        if let Some(first) = self.fonts.first() {
+            first.ascent()
+        } else {
+            self.row_height
+        }
+    }
+
     fn glyph_info_no_cache_or_fallback(&mut self, c: char) -> Option<(FontIndex, GlyphInfo)> {
         for (font_index, font_impl) in self.fonts.iter().enumerate() {
             if let Some(glyph_info) = font_impl.glyph_info(c) {