roundedrect: Fix an intersection cornercase

These 2 rectangles used to intersect fine:
  0 0 50 50 / 50 0
  0 0 50 50 / 0 50
But the computed result was:
  0 0 50 50 / 50
which is not a valid rectangle, because the corners overlap.

Make sure such rectangles return NOT_REPRESENTABLE.

The above rectangle has been added to the testsuite.

gsk/gskroundedrect.c

@@ -46,22 +46,12 @@
 
 #include <math.h>
 
-static void
-gsk_rounded_rect_normalize_in_place (GskRoundedRect *self)
+static float
+gsk_rounded_rect_get_corner_scale_factor (GskRoundedRect *self)
 {
   float factor = 1.0;
   float corners;
-  guint i;
 
-  graphene_rect_normalize (&self->bounds);
-
-  for (i = 0; i < 4; i++)
-    {
-      self->corner[i].width = MAX (self->corner[i].width, 0);
-      self->corner[i].height = MAX (self->corner[i].height, 0);
-    }
-
-  /* clamp border radius, following CSS specs */
   corners = self->corner[GSK_CORNER_TOP_LEFT].width + self->corner[GSK_CORNER_TOP_RIGHT].width;
   if (corners > self->bounds.size.width)
     factor = MIN (factor, self->bounds.size.width / corners);
@@ -78,6 +68,26 @@ gsk_rounded_rect_normalize_in_place (GskRoundedRect *self)
   if (corners > self->bounds.size.height)
     factor = MIN (factor, self->bounds.size.height / corners);
 
+  return factor;
+}
+
+static void
+gsk_rounded_rect_normalize_in_place (GskRoundedRect *self)
+{
+  float factor;
+  guint i;
+
+  graphene_rect_normalize (&self->bounds);
+
+  for (i = 0; i < 4; i++)
+    {
+      self->corner[i].width = MAX (self->corner[i].width, 0);
+      self->corner[i].height = MAX (self->corner[i].height, 0);
+    }
+
+  /* clamp border radius, following CSS specs */
+  factor = gsk_rounded_rect_get_corner_scale_factor (self);
+
   for (i = 0; i < 4; i++)
     graphene_size_scale (&self->corner[i], factor, &self->corner[i]);
 }
@@ -812,7 +822,8 @@ gsk_rounded_rect_intersection (const GskRoundedRect *a,
       check_corner (a, b,
                     GSK_CORNER_BOTTOM_RIGHT,
                     right, bottom,
-                    result))
+                    result) &&
+      gsk_rounded_rect_get_corner_scale_factor (result) >= 1.0)
     return GSK_INTERSECTION_NONEMPTY;
 
   return GSK_INTERSECTION_NOT_REPRESENTABLE;

testsuite/gsk/rounded-rect.c

@@ -346,6 +346,11 @@ test_intersect (void)
       ROUNDED_RECT_INIT_UNIFORM(0, 0, 21, 21, 21, 0, 0, 0),
       GSK_INTERSECTION_NOT_REPRESENTABLE,
     },
+    {
+      ROUNDED_RECT_INIT_UNIFORM(0, 0, 50, 50, 50, 0, 50, 0),
+      ROUNDED_RECT_INIT_UNIFORM(0, 0, 50, 50, 0, 50, 0, 50),
+      GSK_INTERSECTION_NOT_REPRESENTABLE,
+    },
   };
   gsize i;