surface-screenshot: a new manual test module

surface-screenshot: a new manual test module

Add a new Weston plugin under tests/ for manual testing of the
surface-shooting API.

The debug key binding 'h' triggers a surface shot from the surface that
currently has the pointer focus. The shot is written in PAM format into
a file. PAM format was chosen because it is dead-simple to write from
scratch and can carry an RGBA format.

Changes in v2:
- check fprintf calls, fix a malloc without free
- remove stride and format arguments from the API

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
v1 Tested-by: Nobuhiko Tanibata <NOBUHIKO_TANIBATA@xddp.denso.co.jp>
Reviewed-by: Daniel Stone <daniels@collabora.com>