window-x11: Fix a copy/paste error

We need to use the explicitly passed-in gravity here, rather
than the window's existing gravity.
window-x11: Clean up a bit
window-x11: Clean up formatting a bit more
window: Change meta_window_move_resize_internal to take a MetaRectangle

We construct one anyway, and most callers already pass in a rectangle
the long way around, so why not change the internal API?
constraints: Remove a lie

The orig / new now are about the frame rect, not the inner window
rect.
window: Rearrange code a tiny bit
window: Clean up old_rect handling a tiny bit

We need the old rect for two purposes: to find the x/y in a resize-only
action, and to pass into the constraints code for nefarious purposes.

The constraints code takes a frame rectangle, so we convert the original
client rect into a frame rect, but never convert it back since it's
unused for the rest of the function.

Instead of playing games with the variables, just have two,
separately-scoped variables. One is the client rect, the other is the
frame rect.