xoverlay: change new set_render_rectangle() vfunc to take four arguments so we don't depend on libgstvideo

Don't make libgstinterfaces (and thus libgstaudio etc.) indirectly depend
on libgstvideo by using the GstVideoRectangle helper structure in the API,
which causes undesirable dependencies, esp. with the gobject-introspection
(people will point and laugh at us if they find out that libgstaudio
depends on libgstvideo). Instead, pass the x, y, width and height parameters
directly to the function.

Re-fixes #610249.