enabled expressions on x, y, w, h and t parameters for drawgrid and drawbox

I needed expressions to work for drawbox, so I went ahead and implemented it using vf_scale and vf_drawtext as models. The expressions are evaluated once in config_input, so filter_frame is unchanged. The expressions are evaluated 5 times to handle expressions that reference other expressions (example: h=w/dar).

cheers,
-Mark