Mesh rework, part 6: don't rebind default VAO after draw call.

There are a few corner cases namely with binding element buffer, which
should be resolved as soon as possible.

src/Buffer.h

@@ -321,6 +321,8 @@ class MAGNUM_EXPORT Buffer {
          *
          * @todo Allow binding to Target::ElementArray only if VAO is bound
          *      to avoid potential issues?
+         * @bug Binding to ElementArray if any VAO is active will corrupt the mesh
+         * @todo Don't allow user to bind buffers?
          * @see @fn_gl{BindBuffer}
          */
         inline void bind(Target target) { bind(target, _id); }

src/Mesh.cpp

@@ -162,9 +162,7 @@ void Mesh::unbindImplementationDefault() {
 }
 
 #ifndef MAGNUM_TARGET_GLES
-void Mesh::unbindImplementationVAO() {
-    bindVAO(0);
-}
+void Mesh::unbindImplementationVAO() {}
 #endif
 
 }