gsdx-ogl: optimize GPU->CPU memory transfer size

Might help for snow engine game (a little)

Previous code use to read the full texture whereas now it will be limited
to the size of the useful data.

plugins/GSdx/GSTextureCacheOGL.cpp

@@ -83,8 +83,9 @@ void GSTextureCacheOGL::Read(Target* t, const GSVector4i& r)
 	if(GSTexture* offscreen = m_renderer->m_dev->CopyOffscreen(t->m_texture, src, r.width(), r.height(), fmt, ps_shader))
 	{
 		GSTexture::GSMap m;
+		GSVector4i r_offscreen(0, 0, r.width(), r.height());
 
-		if(offscreen->Map(m))
+		if(offscreen->Map(m, &r_offscreen))
 		{
 			// TODO: block level write