missing resources when printing from chromium

[maxnikulin]

Chromium (linux) does not wait for images when Window.print() is invoked.
As a result a grid exported to PDF may look not so pretty as expected.

The issue may be reproduced using tests/test_grid_printer.html
I use echo_sleep option in nginx to simulate some delay (1s)
in loading of tests/support/data/status.png file. It real life
it may happen due to network latency. Sometimes that
delay is not necessary at all.

1. Load test_grid_printer.html page

2. Check the following options:

   • Use CSS File
   • Use Formatters

   and maybe Row Count

3. Click Print

4. Maximize the print popup window (it can be blocked by browser,
   that case enable it)

5. Ensure that "background" is checked in the chromium
   print preview options.

Status icons are absent in the print preview window and it is a bug.

If Window.print() is invoked with some timeout
https://github.com/oria/gridx/blob/master/support/printer.js#L93
then there is no problem with images.

It seems that the export window have to wait for some event (load?).
Unfortunately it can not be done by printer.js since that script
belongs to original page. Maybe a script that initiates print
should be injected into the generated export page.