GL: properly destroy a mesh VAO.

Whoops, this got silently omitted during the massive refactor in
f7a6d79aa0 (Nov 23). Buffers *do* get
destroyed, VAOs not. If you got sudden GPU memory usage issues after a
recent Magnum update, this was why.

doc/changelog.dox

@@ -1035,6 +1035,8 @@ See also:
     @ref GL::Buffer::bind() call as affecting also the regular binding point,
     leading to wrong buffer object being used for data upload etc. in certain
     cases on platforms without DSA extensions.
+-   A large refactor of @ref GL::Mesh internals caused VAOs to no longer be
+    correctly deleted on destruction (see [mosra/magnum#632](https://github.com/mosra/magnum/issues/632))
 -   @ref GL::Context move constructor was not marked @cpp noexcept @ce by
     accident and it was also not really moving everything properly, especially
     when delayed creation was done on the moved-to object

src/Magnum/GL/Mesh.cpp

@@ -344,6 +344,8 @@ Mesh::~Mesh() {
     /* Remove current vao from the state */
     GLuint& current = Context::current().state().mesh.currentVAO;
     if(current == _id) current = 0;
+
+    Context::current().state().mesh.destroyImplementation(*this);
 }
 
 Mesh::Mesh(Mesh&& other) noexcept: _id(other._id), _primitive(other._primitive), _flags{other._flags}, _countSet{other._countSet}, _count(other._count), _baseVertex{other._baseVertex}, _instanceCount{other._instanceCount},