it returns the first found visual for the given depth

if the root background pixmap's depth doesn't match the root window's
depth, find a suitable visual for the root background pixmap's depth and
use it instead of the root window's visual

fix the same issue in the legacy backends, see the previous commit for
details. this commit also removes the x_validate_pixmap function because
it was used only in the get_root_tile function and the fix pretty much
implies and embeds it.