gl renderer: Fix scale computation

We don't need to just look at the scale of the new modelview matrix, but
at the one we get when multiplying the new one with the current one.

Test case attached.

gsk/gl/gskglrenderops.c

@@ -450,7 +450,7 @@ ops_push_modelview (RenderOpBuilder *builder,
   entry->metadata.dx_before = builder->dx;
   entry->metadata.dy_before = builder->dy;
 
-  extract_matrix_metadata (transform, &entry->metadata);
+  extract_matrix_metadata (entry->transform, &entry->metadata);
 
   builder->dx = 0;
   builder->dy = 0;

testsuite/gsk/compare/scale-up-down.node

@@ -0,0 +1,39 @@
+transform{
+  transform: scale(0.5);
+  child: transform {
+    transform: scale(8);
+    child: clip {
+      clip: 0 0 50 50 / 20 0 0 0;
+      child: color {
+        color: teal;
+      }
+    }
+  }
+}
+
+debug {
+  message: "clipped-out area";
+  child: container {
+    color { color: black;
+            bounds: 0 50 10 30; }
+    color { color: black;
+            bounds: 5 40 10 10; }
+    color { color: black;
+            bounds: 10 35 10 10; }
+    color { color: black;
+            bounds: 12 30 10 10; }
+    color { color: black;
+            bounds: 17 23 10 10; }
+    color { color: black;
+            bounds: 23 16 10 10; }
+    color { color: black;
+            bounds: 30 11 10 10; }
+    color { color: black;
+            bounds: 38 4 10 10; }
+    color { color: black;
+            bounds: 47 2 10 10; }
+    color { color: black;
+            bounds: 54 0 50 10; }
+  }
+
+}

testsuite/gsk/meson.build

@@ -56,6 +56,7 @@ compare_render_tests = [
   'texture-url',
   'color-matrix-identity',
   'clip-nested1',
+  'scale-up-down'
 ]
 
 renderers = [