Skia: Enable subpixel glyph positioning to fix uneven text spacing

Without subpixel positioning, Skia rounds each glyph position to the
nearest integer pixel. Since parley provides fractional glyph positions,
this rounding caused visible spacing issues at low resolutions.

The old Skia Paragraph::paint() path handled subpixel positioning
internally, so this was a regression introduced when switching to
parley for text layout.

Fixes slint-ui#10752

Author: tronical

internal/renderers/skia/itemrenderer.rs

@@ -1017,7 +1017,8 @@ impl GlyphRenderer for SkiaItemRenderer<'_> {
         else {
             return;
         };
-        let font = skia_safe::Font::from_typeface(type_face, font_size.get());
+        let mut font = skia_safe::Font::from_typeface(type_face, font_size.get());
+        font.set_subpixel(true);
 
         let (glyph_ids, glyph_positions): (Vec<_>, Vec<_>) = glyphs_it
             .into_iter()