Postpone generation of the RenderTarget ID to RenderTarget::initialize() so that a new ID is generated whenever the RenderTarget is re-create()ed.

src/SFML/Graphics/RenderTarget.cpp

@@ -129,7 +129,7 @@ RenderTarget::RenderTarget() :
 m_defaultView(),
 m_view       (),
 m_cache      (),
-m_id         (getUniqueId())
+m_id         (0)
 {
     m_cache.glStatesSet = false;
 }
@@ -547,6 +547,10 @@ void RenderTarget::initialize()
 
     // Set GL states only on first draw, so that we don't pollute user's states
     m_cache.glStatesSet = false;
+
+    // Generate a unique ID for this RenderTarget to track
+    // whether it is active within a specific context
+    m_id = getUniqueId();
 }