don't resize pane with delta

[yshui]

say the pane with 100 cols wide, we got 3 mouse events, one move the split to 99, the next one move it to 98, then to 90

we would first call resize_split_by with -1, -1, then with -8. the first resize is completed just as we call resize_split_by(-8), this triggers a resync, and updates the split position to 99. because of that resize_split_by(-8) will try to resize it to 91, instead of the desire 90.

in practice, there are a lot more mouse events and the timing is more unpredictable, the end result can be very chaotic.

partially fix #5142

(here is a clip illustrating the problem: https://fosstodon.org/@yshui/113013856546255206)

[yshui]

there are more issues that need to be fixed to resolve #5142

there is a feedback loop: (client) resize -> (server) resize -> (client) resync -> (client) resize.
there is an issue with display size vs real size.

and there is another issue that cause the server to become unresponsive after the client sent it a large number of messages. this one i haven't started looking into yet.