Math: ability to join two ranges.

I had to include Functions.h, there was no other way to do this. I have
to schedule include cleanup soon.

src/Magnum/Math/Range.h

@@ -29,6 +29,7 @@
  * @brief Class @ref Magnum::Math::Range, @ref Magnum::Math::Range2D, @ref Magnum::Math::Range3D, alias @ref Magnum::Math::Range1D
  */
 
+#include "Magnum/Math/Functions.h"
 #include "Magnum/Math/Vector3.h"
 
 namespace Magnum { namespace Math {
@@ -523,6 +524,19 @@ template<class T> class Range3D: public Range<3, T> {
         MAGNUM_RANGE_SUBCLASS_IMPLEMENTATION(3, Range3D, Vector3)
 };
 
+/** @relates Range
+@brief Join two ranges
+
+Returns a range that contains both input ranges. If one of the ranges is empty,
+only the other is returned. Results are undefined if any range has negative
+size.
+*/
+template<UnsignedInt dimensions, class T> inline Range<dimensions, T> join(const Range<dimensions, T>& a, const Range<dimensions, T>& b) {
+    if(a.min() == a.max()) return b;
+    if(b.min() == b.max()) return a;
+    return {min(a.min(), b.min()), max(a.max(), b.max())};
+}
+
 /** @debugoperator{Magnum::Math::Range} */
 template<UnsignedInt dimensions, class T> Corrade::Utility::Debug& operator<<(Corrade::Utility::Debug& debug, const Range<dimensions, T>& value) {
     debug << "Range({" << Corrade::Utility::Debug::nospace << value.min()[0];

src/Magnum/Math/Test/RangeTest.cpp

@@ -123,6 +123,8 @@ struct RangeTest: Corrade::TestSuite::Tester {
     void padded();
     void scaled();
 
+    void join();
+
     void subclassTypes();
     void subclass();
 
@@ -157,6 +159,8 @@ RangeTest::RangeTest() {
               &RangeTest::padded,
               &RangeTest::scaled,
 
+              &RangeTest::join,
+
               &RangeTest::subclassTypes,
               &RangeTest::subclass,
 
@@ -446,6 +450,18 @@ void RangeTest::scaled() {
     CORRADE_COMPARE(a.scaled({2, -3}), b);
 }
 
+void RangeTest::join() {
+    Range2Di a{{12, 20}, {15, 35}};
+    Range2Di b{{10, 25}, {17, 105}};
+    Range2Di c{{130, -15}, {130, -15}};
+    Range2Di d{{10, 20}, {17, 105}};
+
+    CORRADE_COMPARE(Math::join(a, b), d);
+    CORRADE_COMPARE(Math::join(b, a), d);
+    CORRADE_COMPARE(Math::join(a, c), a);
+    CORRADE_COMPARE(Math::join(c, a), a);
+}
+
 template<class T> class BasicRect: public Math::Range<2, T> {
     public:
         /* MSVC 2015 can't handle {} here */