Fix sf::Context::getActiveContext to stop returning inactive contexts

sf::Context::getActiveContext could return an inactive context if
RenderTargets activation occured since the last sf::Context activation.
This is not what its documentation suggest, and was clearly an
unexpected behaviour.

This fix make the function return NULL if the current active context is
not managed by sf::Context (e.g. because it is a RenderTarget).

src/SFML/Window/Context.cpp

@@ -32,7 +32,7 @@
 
 namespace
 {
-    // This per-thread variable holds the current context for each thread
+    // This per-thread variable holds the last activated sf::Context for each thread
     sf::ThreadLocalPtr<sf::Context> currentContext(NULL);
 }
 
@@ -76,7 +76,11 @@ const ContextSettings& Context::getSettings() const
 ////////////////////////////////////////////////////////////
 const Context* Context::getActiveContext()
 {
-    return currentContext;
+    // We have to check that the last activated sf::Context is still active (a RenderTarget activation may have deactivated it)
+    if (currentContext && currentContext->m_context == priv::GlContext::getActiveContext())
+        return currentContext;
+    else
+        return NULL;
 }
 
 

src/SFML/Window/GlContext.cpp

@@ -549,6 +549,13 @@ GlFunctionPointer GlContext::getFunction(const char* name)
 }
 
 
+////////////////////////////////////////////////////////////
+const GlContext* GlContext::getActiveContext()
+{
+    return currentContext;
+}
+
+
 ////////////////////////////////////////////////////////////
 Uint64 GlContext::getActiveContextId()
 {

src/SFML/Window/GlContext.hpp

@@ -157,6 +157,14 @@ public:
     ////////////////////////////////////////////////////////////
     static GlFunctionPointer getFunction(const char* name);
 
+    ////////////////////////////////////////////////////////////
+    /// \brief Get the currently active context
+    ///
+    /// \return The currently active context or NULL if none is active
+    ///
+    ////////////////////////////////////////////////////////////
+    static const GlContext* getActiveContext();
+
     ////////////////////////////////////////////////////////////
     /// \brief Get the currently active context's ID
     ///