GL: properly reset vertex divisor after drawing w/o a VAO.

Otherwise it messes up subsequent draws.

doc/changelog.dox

@@ -98,6 +98,9 @@ See also:
     due to a pointer not being properly initialized after the
     @ref Platform::GlfwApplication::GLConfiguration "GLConfiguration" rework in
     2018.04 (see [mosra/magnum#246](https://github.com/mosra/magnum/pull/246))
+-   Vertex attribute divisor in @ref GL::Mesh::addVertexBufferInstanced() was
+    not properly cleaned up after when @gl_extension{ARB,vertex_array_object}
+    was disabled, causing subsequent draws to misbehave
 
 @subsection changelog-latest-deprecated Deprecated APIs
 

src/Magnum/GL/Mesh.cpp

@@ -746,8 +746,18 @@ void Mesh::bindImplementationVAO() {
 }
 
 void Mesh::unbindImplementationDefault() {
-    for(const AttributeLayout& attribute: *reinterpret_cast<std::vector<AttributeLayout>*>(&_attributes))
+    for(const AttributeLayout& attribute: *reinterpret_cast<std::vector<AttributeLayout>*>(&_attributes)) {
         glDisableVertexAttribArray(attribute.location);
+
+        /* Reset also the divisor back so it doesn't affect  */
+        if(attribute.divisor) {
+            #ifndef MAGNUM_TARGET_GLES2
+            glVertexAttribDivisor(attribute.location, 0);
+            #else
+            (this->*Context::current().state().mesh->vertexAttribDivisorImplementation)(attribute.location, 0);
+            #endif
+        }
+    }
 }
 
 void Mesh::unbindImplementationVAO() {}