gl: Change rounded_rect_shrink()

The code now follows gsk_rounded_rect_shrink() and with it the behavior
of the Cairo renderer and Webkit.

The old code did what the GL renderer and Cairo do, but I consider that
wrong.

I did not test Chrome.

Test attached

gsk/gl/resources/preamble.vs.glsl

@@ -28,6 +28,8 @@ gsk_rounded_rect_shrink (GskRoundedRect r, vec4 amount)
   if (r.corner_points1.zw == r.bounds.zy) new_corner_points1.zw = new_bounds.zy;
   if (r.corner_points2.xy == r.bounds.zw) new_corner_points2.xy = new_bounds.zw;
   if (r.corner_points2.zw == r.bounds.xw) new_corner_points2.zw = new_bounds.xw;
+  new_corner_points1 = clamp (new_corner_points1, new_bounds.xyxy, new_bounds.zwzw);
+  new_corner_points2 = clamp (new_corner_points2, new_bounds.xyxy, new_bounds.zwzw);
 
   return GskRoundedRect (new_bounds, new_corner_points1, new_corner_points2);
 }

testsuite/gsk/compare/shrink-rounded-border.node

@@ -0,0 +1,19 @@
+/* We clip out the transparent region inside the
+   border so we can check that the shrinking of
+   the inside works properly.
+   Comment out the clip to see the actual border
+ */
+
+clip {
+  /* blame sqrt(2) for those numbers:
+     The shrunkrect should be 40 40 70 70 / 70 0
+     70/sqrt(2) = 49.4974747...
+     40 + 49 = 89 and 40 + 70 - 49 = 61,
+     giving you:
+   */
+  clip: 61 61 28 28;
+  child: border {
+    outline: 0 0 150 150 / 150 0 150 0;
+    widths: 40;
+  }
+}

testsuite/gsk/meson.build

@@ -94,6 +94,7 @@ compare_render_tests = [
   'shadow-behind',
   'shadow-in-opacity',
   'shadow-opacity',
+  'shrink-rounded-border',
   'stroke',
   'texture-scale-magnify-10000x',
   'texture-scale-magnify-rotate',