f32: handle empty rectangles in Union and Intersect

The old renderer depends on the old behaviour of Union, so change that
reference to a copy.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/clip.go

@@ -93,5 +93,22 @@ func (qs *quadSplitter) splitAndEncode(quad stroke.QuadSegment) {
 		}
 	}
 
-	qs.bounds = qs.bounds.Union(cbnd)
+	qs.bounds = unionRect(qs.bounds, cbnd)
+}
+
+// Union is like f32.Rectangle.Union but ignores empty rectangles.
+func unionRect(r, s f32.Rectangle) f32.Rectangle {
+	if r.Min.X > s.Min.X {
+		r.Min.X = s.Min.X
+	}
+	if r.Min.Y > s.Min.Y {
+		r.Min.Y = s.Min.Y
+	}
+	if r.Max.X < s.Max.X {
+		r.Max.X = s.Max.X
+	}
+	if r.Max.Y < s.Max.Y {
+		r.Max.Y = s.Max.Y
+	}
+	return r
 }