BLI: Math: Add xy() and xyz() swizzle functions

Thoses are added for better component masking syntax.
This avoids the harder to read `float2(my_vec4)`.

This is not meant to be fully usable swizzling support but could be
extended in the future.

source/blender/blenlib/BLI_math_vector_types.hh

@@ -174,6 +174,18 @@ template<typename T, int Size> struct VecBase : public vec_struct_base<T, Size>
     }
   }
 
+  /** Swizzling. */
+
+  template<BLI_ENABLE_IF_VEC(Size, >= 3)> VecBase<T, 2> xy() const
+  {
+    return *reinterpret_cast<VecBase<T, 2> *>(&x);
+  }
+
+  template<BLI_ENABLE_IF_VEC(Size, >= 4)> VecBase<T, 3> xyz() const
+  {
+    return *reinterpret_cast<VecBase<T, 3> *>(&x);
+  }
+
 #undef BLI_ENABLE_IF_VEC
 
   /** Conversion from pointers (from C-style vectors). */