cairo: Handle clipped blur content

This is the result of experimenting with corner cases when blurring.

The result is a test that tests when the child of a blur node is
clipped out but the blurred child is not, the blurred parts are still
visible.

This immediately broke the cairo renderer, so the fix is included.

gsk/gskrendernodeimpl.c

@@ -5901,35 +5901,42 @@ gsk_blur_node_draw (GskRenderNode *node,
                     cairo_t       *cr)
 {
   GskBlurNode *self = (GskBlurNode *) node;
-  cairo_pattern_t *pattern;
   cairo_surface_t *surface;
-  cairo_surface_t *image_surface;
+  cairo_t *cr2;
+  graphene_rect_t blur_bounds;
+  double clip_radius;
 
-  cairo_save (cr);
+  clip_radius = gsk_cairo_blur_compute_pixels (0.5 * self->radius);
 
-  /* clip so the push_group() creates a smaller surface */
-  gsk_cairo_rectangle (cr, &node->bounds);
-  cairo_clip (cr);
+  /* We need to extend the clip by the blur radius
+   * so we can blur pixels in that region */
+  _graphene_rect_init_from_clip_extents (&blur_bounds, cr);
+  graphene_rect_inset (&blur_bounds, - clip_radius, - clip_radius);
+  if (!gsk_rect_intersection (&blur_bounds, &node->bounds, &blur_bounds))
+    return;
 
-  cairo_push_group (cr);
+  surface = cairo_surface_create_similar_image (cairo_get_target (cr),
+                                                CAIRO_FORMAT_ARGB32,
+                                                ceil (blur_bounds.size.width),
+                                                ceil (blur_bounds.size.height));
+  cairo_surface_set_device_offset (surface,
+                                   - blur_bounds.origin.x,
+                                   - blur_bounds.origin.y);
 
-  gsk_render_node_draw (self->child, cr);
+  cr2 = cairo_create (surface);
+  gsk_render_node_draw (self->child, cr2);
+  cairo_destroy (cr2);
 
-  pattern = cairo_pop_group (cr);
-  cairo_pattern_get_surface (pattern, &surface);
-  image_surface = cairo_surface_map_to_image (surface, NULL);
-  blur_image_surface (image_surface, (int)self->radius, 3);
+  blur_image_surface (surface, (int) ceil (0.5 * self->radius), 3);
   cairo_surface_mark_dirty (surface);
-  cairo_surface_unmap_image (surface, image_surface);
 
-  cairo_set_source (cr, pattern);
+  cairo_set_source_surface (cr, surface, 0, 0);
   cairo_rectangle (cr,
                    node->bounds.origin.x, node->bounds.origin.y,
                    node->bounds.size.width, node->bounds.size.height);
   cairo_fill (cr);
 
-  cairo_restore (cr);
-  cairo_pattern_destroy (pattern);
+  cairo_surface_destroy (surface);
 }
 
 static void

testsuite/gsk/compare/blur-contents-outside-of-clip.node

@@ -0,0 +1,40 @@
+color-matrix {
+  matrix: matrix3d(256, 0, 0, 0, 0, 256, 0, 0, 0, 0, 256, 0, 0, 0, 0, 256);
+  child: clip {
+    clip: -37 59 50 50;
+    child: container {
+      blur {
+        blur: 17;
+        child: color {
+          bounds: -87 59 50 50;
+          color: rgb(255,0,0);
+        }
+      }
+      blur {
+        blur: 17;
+        child: color {
+          bounds: 13 59 50 50;
+          color: rgb(255,0,0);
+        }
+      }
+      blur {
+        blur: 17;
+        child: color {
+          bounds: -37 9 50 50;
+          color: rgb(255,0,0);
+        }
+      }
+      blur {
+        blur: 17;
+        child: color {
+          bounds: -37 109 50 50;
+          color: rgb(255,0,0);
+        }
+      }
+      color {
+        bounds: -22 74 20 20;
+        color: rgb(0,0,0);
+      }
+    }
+  }
+}

testsuite/gsk/meson.build

@@ -20,6 +20,7 @@ compare_render_tests = [
   'blend-in-rotate',
   'blend-normal',
   'blurred-lines',
+  'blur-contents-outside-of-clip',
   'border-bottom-right',
   'border-one-rounded',
   'borders-rotated',