eglglessink: Disable vertex attribute arrays after drawing

Enable vertex attribute arrays which will be used by a GL program
and disable them after drawing. If a vertex attribute array not
required by a program is enabled, GL will try to read from it,
which may crash the application.

https://bugzilla.gnome.org/show_bug.cgi?id=730912
eglglessink: Set window background as None

Prevent X from redrawing the background on ConfigureNotify by
setting background as None. Otherwise flickering is observed
when resizing the window.

https://bugzilla.gnome.org/show_bug.cgi?id=730981
shmsink: Allocate enough memory to do alignment

https://bugzilla.gnome.org/show_bug.cgi?id=731093
tests: Take account of memory alignment in shm test