mesa-demos: Build with system data files

Use the "--with-system-data-files" when configuring mesa-demos
so that the data files in /usr/share/mesa-demos/ will be properly
located at run time.

Copy some additional data files into the target filesystem.

Signed-off-by: Drew Moseley <drew_moseley@mentor.com>
mesa-demos: Add missing data files.

Add some data files that are present in the git repository:
   http://cgit.freedesktop.org/mesa/demos/tree/?id=mesa-demos-8.1.0
but not in the release tarball
   ftp://ftp.freedesktop.org/pub/mesa/demos/8.1.0/mesa-demos-8.1.0.tar.bz2

Signed-off-by: Drew Moseley <drew_moseley@mentor.com>
mesa-demos: Use DEMOS_DATA_DIR.

Modify mesa demo apps to respect the DEMOS_DATA_DIR
configuration parameter for locating data files.

Signed-off-by: Drew Moseley <drew_moseley@mentor.com>