GL: restore backwards compatibility of deprecated Buffer::map<T>().

Calling map<void>() was trying to instantiate ArrayView<void> through
arrayCast<void>(), which is not possible (void has no size etc etc.).
Casting the returned pointer directly instead. Thanks to @AndySomogyi
for pointing this out.

doc/changelog.dox

@@ -167,6 +167,9 @@ See also:
     optional, defaults to @ref GL::BufferUsage::StaticDraw
 -   Re-enabled @fn_gl{DrawRangeElements} for WebGL 2 (see
     [mosra/magnum#97](https://github.com/mosra/magnum/issues/97))
+-   Restored backwards compatibility to the templated @ref GL::Buffer::map()
+    overload --- it was not possible to call it with @cpp void @ce template
+    parameter
 
 @subsubsection changelog-latest-changes-math Math library
 

src/Magnum/GL/Buffer.h

@@ -1167,7 +1167,7 @@ class MAGNUM_GL_EXPORT Buffer: public AbstractObject {
          *      instead.
          */
         template<class T> CORRADE_DEPRECATED("use non-templated map() and Containers::arrayCast() instead") T* map(GLintptr offset, GLsizeiptr length, MapFlags flags) {
-            return Containers::arrayCast<T>(map(offset, length, flags));
+            return reinterpret_cast<T*>(map(offset, length, flags).data());
         }
         #endif