bug#44140: 26.3; ERC stamps: Really use latest buffer's window's width prior to `fill-column'

[Olivier Certner <olivier.certner@free.fr>]

Applies to 26.3, but also all more recent versions as well.

Bug trigger:
1. Load ERC (e.g., open some IRC connection).
2. In some ERC buffer, deactivate ERC fill mode (if not deactivated
globally in your configuration), e.g., M-x erc-fill-mode.
3. Notice that `fill-column' (if non-nil; if nil, set it temporarily to
some small value, e.g., 40, to see the effect) will be used in this
case, contrary to what the documentation of `erc-insert-timestamp-right'
says (window's width is supposed to be used in priority when
erc-fill-mode is not activated).
4. Set `fill-column' to nil. Then, move to another window selecting
another buffer, whose size is visibly smaller. Wait for messages to
arrive in the ERC buffer and observe the timestamp position: It is
relative to the currently selected window's width!

Root causes:
1. Precedence of window's width over `fill-column' not implemented,
contrary to what the documentation states, which makes much more sense.
2. `window-width' is called to obtain window's width, but this gets the
width of the selected window, which is not necessarily where the buffer
is actually displayed. Moreover, the buffer may not be displayed at the
moment, so some other fallback is necessary.

Patch:
To be attached shortly after bug creation.

-- 
Olivier Certner