Prism/Transparency effect does not work with pre-calculated backgrounds

[ABelliqueux]

With pre-rendered background, we could just set the tpage to bpp, x, y of the image in vram : 8bpp, 320, 0 by default for now.

if (fixed_BGs){
     ( (POLY_GT3 *) poly )->tpage = getTPage( 0, 0, 320, 0 );
}

But how do we now fixed BGs are used ?
We could use curLvl.camPtr->BGtim to determine the existence of the data, and even use prect to know x,y coords in vram,

if (curLvl.camPtr->BGtim){
     ( (POLY_GT3 *) poly )->tpage = getTPage( curLvl.camPtr->BGtim&0x3, 0, 
                                              curLvl.camPtr->BGtim->prect->x,
                                              curLvl.camPtr->BGtim->prect->y );
}

but do we want to add a function argument just for that ?