Add COpenGLCoreCacheHandler::getDepthTest (thx@ Criss)

While COpenGLCoreCacheHandler wasn't meant to be accessed externally, it is possible with some hacks (a few casts and including internal headers) and sometimes necessary.
And that's one of the state-flags people have a need to know from outside the engine.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@5814 dfc29bdd-3216-0410-991c-e03cc46cb475

source/Irrlicht/COpenGLCoreCacheHandler.h

@@ -495,6 +495,11 @@ public:
 		}
 	}
 
+    void getDepthTest(bool& enable)
+    {
+        enable = DepthTest;
+    }
+
 	void setDepthTest(bool enable)
 	{
 		if (DepthTest != enable)