MathUtil: Fix Rectangle::GetWidth/Height for unsigned types

`std::abs(x - y)` where x and y are unsigned integers fails to compile
with an "call of overloaded 'abs(unsigned int)' is ambiguous" error
on GCC, and even if it did compile, that expression still wouldn't
give the correct result since `x - y` is unsigned.

Source/Core/Common/MathUtil.h

@@ -6,6 +6,7 @@
 
 #include <algorithm>
 #include <cmath>
+#include <type_traits>
 #include <vector>
 
 #include "Common/CommonTypes.h"
@@ -71,8 +72,8 @@ struct Rectangle
     return left == r.left && top == r.top && right == r.right && bottom == r.bottom;
   }
 
-  T GetWidth() const { return std::abs(right - left); }
-  T GetHeight() const { return std::abs(bottom - top); }
+  constexpr T GetWidth() const { return GetDistance(left, right); }
+  constexpr T GetHeight() const { return GetDistance(top, bottom); }
   // If the rectangle is in a coordinate system with a lower-left origin, use
   // this Clamp.
   void ClampLL(T x1, T y1, T x2, T y2)
@@ -92,6 +93,15 @@ struct Rectangle
     top = std::clamp(top, y1, y2);
     bottom = std::clamp(bottom, y1, y2);
   }
+
+private:
+  constexpr T GetDistance(T a, T b) const
+  {
+    if constexpr (std::is_unsigned<T>())
+      return b > a ? b - a : a - b;
+    else
+      return std::abs(b - a);
+  }
 };
 
 template <typename T>