Force OpenGL states to be reset when RenderTarget detects that a new …

…context has to be tracked. In the case a context is re-created, FBO RenderTextures would not trigger resetting of the OpenGL states if they were drawn to first since re-initialization of the RenderTarget only happens when a context owner such as a RenderWindow is re-created. Fixes #1625.

src/SFML/Graphics/RenderTarget.cpp

@@ -405,6 +405,7 @@ bool RenderTarget::setActive(bool active)
             {
                 contextRenderTargetMap[contextId] = m_id;
 
+                m_cache.glStatesSet = false;
                 m_cache.enable = false;
             }
             else if (iter->second != m_id)