shadow capture is blank on Ubuntu 16.04

[Antoine Martin]

No errors or warnings, but the resulting capture is blank.

r21099 adds a simple test script for exercising the shadow capture code. (with minor fixups in r21100 and r21101)

All distributions can save a PNG screen capture, except for Ubuntu 16.04

[Antoine Martin]

Re-tested all other current Ubuntu versions (14.04, 16.04, 18.04 and 18.10), only 16.04 is affected.
Fixed a bug when xshm is disabled: #2143, but disabling xshm does not help.
Using the GTK root window capture code tool added in r21645 does not work either.

Strange thing is that xwd does capture the X11 display without problems.

Anyway, turning off "3d acceleration" in virtualbox fixed the problem. Clearly a driver bug.

[migration script]

this ticket has been moved to: ​https://github.com/Xpra-org/xpra/issues/2063