Fix: window doesn't resize after `set lines`, `set columns`

[bprb]

Detect a grid change and resize the (win32) window. Neovim might not send a flush so manually force a full grid redraw.

[bprb]

To repro:

1. start nvy with --geometry 100x20
2. use set lines to verify line count is 20
3. use set lines=30 to increase nvy window height
4. observe that GUI window doesn't resize, but the nvim window does, so the status bar & cmd area are invisible

[RMichelsen]

Sorry for the slow response and thanks for the repro, I am not sure why neovim is not sending a flush but in any case I don't see any harm in a few extra manual flushes to make sure things like this work. Merging 👍