Add a DefaultInit tag.

This one will overload the meaning of "default", so updating the docs
accordingly.

doc/changelog.dox

@@ -41,6 +41,8 @@ See also:
 @subsection changelog-latest-new New features
 
 -   New @ref NoAllocate constructor tag, to be used by the @ref Vk library
+-   New @ref DefaultInit constructor tag, simply an alias to the existing
+    @ref Corrade::DefaultInit tag
 -   New @ref PixelFormat::Depth16Unorm, @relativeref{PixelFormat,Depth24Unorm},
     @relativeref{PixelFormat,Depth32F}, @relativeref{PixelFormat,Stencil8UI},
     @relativeref{PixelFormat,Depth16UnormStencil8UI},

src/Magnum/Tags.h

@@ -35,6 +35,21 @@
 
 namespace Magnum {
 
+/**
+@brief Default initialization tag type
+@m_since_latest
+
+Used to distinguish construction with default initialization. The actual
+meaning of "default" may vary, see documentation of a particular API using this
+tag for a detailed behavior description.
+@see @ref DefaultInit
+*/
+#ifdef DOXYGEN_GENERATING_OUTPUT
+typedef Corrade::DefaultInitT DefaultInitT;
+#else
+using Corrade::DefaultInitT;
+#endif
+
 /**
 @brief No initialization tag type
 @m_since{2020,06}
@@ -76,6 +91,20 @@ struct NoAllocateT {
     #endif
 };
 
+/**
+@brief Default initialization tag
+@m_since_latest
+
+Use for construction with default initialization. The actual meaning of
+"default" may vary, see documentation of a particular API using this tag for
+a detailed behavior description.
+*/
+#ifdef DOXYGEN_GENERATING_OUTPUT
+constexpr DefaultInitT DefaultInit{};
+#else
+using Corrade::DefaultInit;
+#endif
+
 /**
 @brief No initialization tag
 @m_since{2020,06}