Add subpixel_binning to TextOptions and FontTweak (#8072)

This lets you turn off subpixel horizontal binning of glyphs. The option
is a trade-off between even kerning and sharp text.

* Closes https://github.com/emilk/egui/issues/8034

crates/epaint/src/text/mod.rs

@@ -34,6 +34,20 @@ pub struct TextOptions {
     ///
     /// Default is `true`.
     pub font_hinting: bool,
+
+    /// Enable sub-pixel binning for glyphs.
+    ///
+    /// Sub-pixel binning renders each glyph at up to four fractional horizontal offsets,
+    /// giving more even kerning at the cost of more atlas space.
+    ///
+    /// It also lead to text looking more blurry.
+    ///
+    /// This is always disabled for CJK characters (which have too many unique glyphs).
+    ///
+    /// Can be overridden per font with [`FontTweak::subpixel_binning`].
+    ///
+    /// Default: `true`.
+    pub subpixel_binning: bool,
 }
 
 impl Default for TextOptions {
@@ -42,6 +56,7 @@ impl Default for TextOptions {
             max_texture_side: 2048, // Small but portable
             alpha_from_coverage: crate::AlphaFromCoverage::default(),
             font_hinting: true,
+            subpixel_binning: true,
         }
     }
 }