[arm64] mclient crashes when changing display resolution the first time

Steps to repro:

0. Boot arm64 jessie container on bullhead
0. Cast to chromecast
0. Use Settings > Display to change the resolution from the default 1920x1080 to 1280x720

Screen will go black and logs show:
```
BadValue (integer paramater out of range for operation): request code 130
error calling XShmGetImage
error calling XShmGetImage
...

```

The timeout on the resolution change triggers and screen switches back to default 1920x1080, then mclient segfaults and restarts. Then you can change the display resolution just fine! Weird.

I can't reproduce this on armhf.

Start by reproducing the arm64 setup on bullhead: boot the arm64 Jessie container, cast to Chromecast, and change the display from 1920x1080 to 1280x720. Trace mclient's resolution-change path around the XShmGetImage errors; the issue is done when the first resolution change no longer times out, segfaults, or restarts mclient.

Written by the indexing model from the issue text.