fix first run of shader-using 3D transitions

The slideshow engine emits viewChanged() immediately after the
transition has been initialized. Because the ShaderTransition data were
not fully reset in finishTransition_, no transition texture would be
generated on the re-initialization in viewChanged(), thus there would be
no transition on the first slide.

Change-Id: I04fef9a6c09537995d83889be2fe15569a1ddc2c

slideshow/source/engine/OGLTrans/unx/OGLTrans_TransitionImpl.cxx

@@ -1449,6 +1449,11 @@ void ShaderTransition::finishTransition_()
         OGLShaders::glDeleteProgram( m_nProgramObject );
         m_nProgramObject = 0;
     }
+    if ( m_nHelperTexture )
+    {
+        glDeleteTextures( 1, &m_nHelperTexture );
+        m_nHelperTexture = 0;
+    }
 #endif
 }