LibGfx: Make Matrix and VectorN more constexpr-friendly

This allows the copy constructor of Matrix to be called constexpr,
which should allow more values to be compile-time calculated.

Likewise, VectorN.data() is now constexpr so that it can be compile-time
evaluated.

Userland/Libraries/LibGfx/Matrix.h

@@ -36,13 +36,23 @@ public:
     {
     }
 
-    Matrix(Matrix const& other)
+    constexpr Matrix(Matrix const& other)
     {
-        __builtin_memcpy(m_elements, other.elements(), sizeof(T) * N * N);
+        *this = other;
     }
 
-    Matrix& operator=(Matrix const& other)
+    constexpr Matrix& operator=(Matrix const& other)
     {
+#ifndef __clang__
+        if (is_constant_evaluated()) {
+            for (size_t i = 0; i < N; i++) {
+                for (size_t j = 0; j < N; j++) {
+                    m_elements[i][j] = other.elements()[i][j];
+                }
+            }
+            return *this;
+        }
+#endif
         __builtin_memcpy(m_elements, other.elements(), sizeof(T) * N * N);
         return *this;
     }

Userland/Libraries/LibGfx/VectorN.h

@@ -251,8 +251,8 @@ public:
         return result;
     }
 
-    auto& data() { return m_data; }
+    constexpr auto& data() { return m_data; }
-    auto const& data() const { return m_data; }
+    constexpr auto const& data() const { return m_data; }
 
 private:
     AK::Array<T, N> m_data;