compositor: merge surface size from buffer size funcs

compositor: merge surface size from buffer size funcs

Replace the two functions getting the intended surface dimensions from
the surface's buffer and buffer transformation parameters by a single
function that just set the surface size according to all the buffer
state.

The old functions were always called in pairs, and always assigned to
the surface dimension variables.

This function also deals with a NULL buffer by setting the dimensions to
zero, just like the callers used to do.

The new function has no users outside this source file, so do not export
it. This basically unexports the old functions.