weston: Add config option to enable pixman-based rendering

Pixman can be used for rendering if the default GLESv2 rendering
is broken or cannot be used.

Pixman-based rendering is already available with the command-line
switch '--use-pixman'. This patch adds support for this option to
the configuration file. Putting

  [core]
  use-pixman=true

into 'weston.ini' enables pixman-based rendering for all backends
that support it. With this change, pixman has to be enabled only
once.

Fixes: https://gitlab.freedesktop.org/wayland/weston/issues/27


Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>