GL: ensure the texture is actually created when using ARB_multi_bind.

With ARB_DSA disabled, the test added in previous commit would assert
already during bind(), and not later in label(). This was due to the
state tracker being updated too early here.

src/Magnum/GL/AbstractTexture.cpp

@@ -347,9 +347,13 @@ void AbstractTexture::bind(Int textureUnit) {
     /* If already bound in given texture unit, nothing to do */
     if(textureState.bindings[textureUnit].second() == _id) return;
 
-    /* Update state tracker, bind the texture to the unit */
+    /* Bind the texture to the unit, *then* update the state tracker. The order
-    textureState.bindings[textureUnit] = {_target, _id};
+       is important, as if bindImplementationMulti() is used, it calls into
+       createIfNotAlready() which then, if the state tracker would be already
+       updated, would see that the texture is bound already, and thus wouldn't
+       bind it at all, never actually creating it. */
     textureState.bindImplementation(*this, textureUnit);
+    textureState.bindings[textureUnit] = {_target, _id};
 }
 
 void AbstractTexture::bindImplementationDefault(AbstractTexture& self, GLint textureUnit) {