gsdx: extend CopyOffscreen with a new shader parameter

Currently we're trying to infer the conversion shader based on the output format

It only works if the input data is RGBA8. It might not be true in the future

plugins/GSdx/GSDeviceSW.cpp

@@ -90,7 +90,7 @@ void GSDeviceSW::ClearStencil(GSTexture* t, uint8 c)
 	Clear(t, c);
 }
 
-GSTexture* GSDeviceSW::CopyOffscreen(GSTexture* src, const GSVector4& sRect, int w, int h, int format)
+GSTexture* GSDeviceSW::CopyOffscreen(GSTexture* src, const GSVector4& sRect, int w, int h, int format, int ps_shader)
 {
 	GSTexture* dst = CreateOffscreen(w, h, format);