ERC right stamps: also use latest buffer's window's width (Bug#44140)

ERC right stamps: also use latest buffer's window's width (Bug#44140)

* lisp/erc/erc-stamp.el (erc-insert-timestamp-right): Use latest
buffer's window's width to position the timestamp, if both
`erc-timestamp-right-column' and `erc-fill-column' are not set (or
`erc-fill-mode' is off).  This is what the documentation says, but was
not implemented.  Also fix the bug of using selected window's width
instead of the (or some) window showing the buffer.  The latest
window's width is saved in `erc-timestamp-last-window-width' and used
when the buffer is no more shown.  In case the buffer was never shown,
which I'm not sure can happen, either use `fill-column' if set, or
give up on aligning and just output the timestamp (modulo the kludge)
right after message text.  While here, fix the off by one calculation
of point start when the reference is the window's width.