MGenericPtr Xpos.w,Ypos.w,BaseAddress.l,RowWidth.w ; Calculate and return address based on inputs

Returns an address based on the parameters specified. Xpos,Ypos are coordinates
which may represent the offset within a bitmap or something. The expression for
working out the value to return is Base=BaseAddress+(Ypos*RowWidth)+Xpos.
RowWidth is therefore needed to work out the width of the image for creating the
proper offset for the Xpos,Ypos coordinate. If using this command with regards to
planar bitmaps (blitz's normal Bitmap objects) for example, you would need to
devide Xpos and RowWidth by 8 before passing the parameters.