gsk: Fix ink leakage in node diffing

We do need to enlarge the node bounds for text nodes after all,
to prevent leaking ink, in particular with fractional scaling.

This reverts 24de5ffd4e.

gsk/gpu/gskgpunodeprocessor.c

@@ -3032,12 +3032,19 @@ gsk_gpu_node_processor_add_glyph_node (GskGpuNodeProcessor *self,
                                           offset.y + (float) glyphs[i].geometry.y_offset / PANGO_SCALE);
       if (glyph_align)
         {
-          glyph_origin.x = roundf (glyph_origin.x * scale * 4);
-          glyph_origin.y = roundf (glyph_origin.y * scale * 4);
-          flags = ((int) glyph_origin.x & 3) |
-                  (((int) glyph_origin.y & 3) << 2);
-          glyph_origin.x = 0.25 * inv_scale * glyph_origin.x;
-          glyph_origin.y = 0.25 * inv_scale * glyph_origin.y;
+          float x, y;
+          int frac_x, frac_y;
+
+          x = glyph_origin.x * scale;
+          y = glyph_origin.y * scale;
+
+          frac_x = (int) roundf ((x - floor (x)) * 4);
+          frac_y = (int) roundf ((y - floor (y)) * 4);
+
+          flags = (frac_x & 3) | ((frac_y & 3) << 2);
+
+          glyph_origin.x = (floor (x) + (frac_x == 4 ? 1.f : 0.f)) * inv_scale;
+          glyph_origin.y = (floor (y) + (frac_y == 4 ? 1.f : 0.f)) * inv_scale;
         }
       else
         {

gsk/gskrendernodeimpl.c

@@ -5854,11 +5854,19 @@ gsk_text_node_new (PangoFont              *font,
   self->glyphs = glyph_infos;
   self->num_glyphs = n;
 
+  /* Since we do subpixel positioning in device pixels, and the app and
+   * device pixel grid may not be aligned, we can end up rounding down
+   * for subpixel positioning even when we are exactly on an app pixel
+   * boundary.
+   *
+   * Play it safe, and enlarge the node bounds by one app pixel
+   * in each direction, so we don't leak ink when diffing.
+   */
   gsk_rect_init (&node->bounds,
-                 offset->x + ink_rect.x,
-                 offset->y + ink_rect.y,
-                 ink_rect.width,
-                 ink_rect.height);
+                 offset->x + ink_rect.x - 1,
+                 offset->y + ink_rect.y - 1,
+                 ink_rect.width + 2,
+                 ink_rect.height + 2);
 
   return node;
 }