Fix uneven text kerning for non-integral dpi scales

Closes https://github.com/emilk/egui/issues/382

epaint/src/tessellator.rs

@@ -645,6 +645,13 @@ impl Tessellator {
             galley.sanity_check();
         }
 
+        // The contents of the galley is already snapped to pixel coordinates,
+        // but we need to make sure the galley ends up on the start of a physical pixel:
+        let pos = pos2(
+            self.options.round_to_pixel(pos.x),
+            self.options.round_to_pixel(pos.y),
+        );
+
         let num_chars = galley.char_count_excluding_newlines();
         out.reserve_triangles(num_chars * 2);
         out.reserve_vertices(num_chars * 4);

epaint/src/text/font.rs

@@ -80,7 +80,13 @@ impl FontImpl {
 
         let scale_in_pixels = pixels_per_point * scale_in_points;
 
+        // Round to an even number of physical pixels to get even kerning.
+        // See https://github.com/emilk/egui/issues/382
+        let scale_in_pixels = scale_in_pixels.round();
+        let scale_in_points = scale_in_pixels / pixels_per_point;
+
         let height_in_points = scale_in_points;
+
         // TODO: use v_metrics for line spacing ?
         // let v = rusttype_font.v_metrics(Scale::uniform(scale_in_pixels));
         // let height_in_pixels = v.ascent - v.descent + v.line_gap;