scripts: run-demo-random.sh: use only POSIX sh syntax
scripts: run-demo-random.sh: use only POSIX sh syntax

Also test if "timeout" command knows the "--foreground" option,
which is present is coreutils. This allow this script to run with
the "timeout" command included in Busybox, which does not support
this option.

Also use the new "ls-shaders" script instead for listing and sorting the
shaders to show.

Signed-off-by: Julien Olivain <7fdfe229fce69a4d7f38653f3755ccbae5703f88@cotds.org>

jolivain/glslsandbox-player,jolivain/glslsandbox-player,jolivain/glslsandbox-player,jolivain/glslsandbox-player