Fixed alpha-blended drawables getting incorrect alpha values when rendered through a RenderImage

Replaced internal calls to glewIsSupported with the corresponding constants (faster checks)

git-svn-id: https://sfml.svn.sourceforge.net/svnroot/sfml/branches/sfml2@1264 4e206d99-4929-0410-ac5d-dfc041789085

src/SFML/Graphics/GeometryRendererVBO.cpp

@@ -37,7 +37,7 @@ bool GeometryRendererVBO::IsSupported()
 {
     priv::EnsureGlewInit();
 
-    return glewIsSupported("GL_ARB_vertex_buffer_object") != 0;
+    return GLEW_ARB_vertex_buffer_object != 0;
 }