MathUtil: Make Rectangle work with unsigned types.

src/common/math_util.h

@@ -7,6 +7,7 @@
 #include "common/common.h"
 
 #include <algorithm>
+#include <type_traits>
 #include <vector>
 
 namespace MathUtil
@@ -112,8 +113,8 @@ struct Rectangle
 
     bool operator==(const Rectangle& r) { return left==r.left && top==r.top && right==r.right && bottom==r.bottom; }
 
-    T GetWidth() const { return abs(right - left); }
-    T GetHeight() const { return abs(bottom - top); }
+    T GetWidth() const { return std::abs(static_cast<typename std::make_signed<T>::type>(right - left)); }
+    T GetHeight() const { return std::abs(static_cast<typename std::make_signed<T>::type>(bottom - top)); }
 
     // If the rectangle is in a coordinate system with a lower-left origin, use
     // this Clamp.