Clear the entire window when the xim size is potentially changed.

This fixes issues where "leftovers" from the previous mode would
remain in a new screen mode.
14 jobs for master in 7 minutes and 31 seconds (queued for 9 seconds)
Status Job ID Name Coverage
  Build
passed #23446
FreeBSD
jsdoor-freebsd

00:05:22

passed #23447
Linux
jsdoor-linux

00:05:29

passed #23449
FreeBSD
jsdoor-windows

00:02:39

passed #23441
FreeBSD
sbbs-freebsd

00:07:31

passed #23436
Linux
sbbs-linux

00:04:58

passed #23439
Windows
sbbs-windows

00:05:21

passed #23448
FreeBSD
sexpots-freebsd

00:00:11

passed #23437
Linux
sexpots-linux

00:00:22

passed #23440
Windows
sexpots-windows

00:00:25

passed #23444
FreeBSD
syncdraw-freebsd

00:00:29

passed #23445
Linux
syncdraw-linux

00:00:20

passed #23442
FreeBSD
syncterm-freebsd

00:01:25

passed #23438
Linux
syncterm-linux

00:02:15

passed #23443
FreeBSD
syncterm-windows

00:02:24