XvfbDriver should set up Xvfb instances providing screens of 8-bit depth
https://bugs.webkit.org/show_bug.cgi?id=121951

Reviewed by Carlos Garcia Campos.

Screens with 24-bit depth are causing problems as it doesn't seem to be possible to fall back to
software rendering successfully on the GTK 64-bit release builder, resulting in crashes in almost
500 layout tests that trigger accelerated compositing to be used.

* Scripts/webkitpy/port/xvfbdriver.py:
(XvfbDriver._start):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@156457 268f45cc-cd09-0410-ab3c-d52691b4dbfc