Several window resizing operations (i.e., --auto-window-resize,
border dragging with --keep-aspect-window, Alt+0/1/2) cause the
window to detach from the snapped borders. This patch resolves
this by recording to which borders the window is snapped, and
using this info to determine how the window should be placed after
resizing.
Closes https://github.com/mpv-player/mpv/issues/6588