Needs testing on Win32 with Cygwin

Code cleanup without semantics changes
Resets the reference count when returning a previously allocated picture
Check error path

--
Rafaël Carré <rcarre@m2x.nl>

commit dc9057ec9b4bb41a74bb07197c64cee8c1976fe0
Author: Rafaël Carré <funman@videolan.org>
Date:   Mon May 19 10:08:04 2008 +0200

    Do not install swscale headers if it was not enabled
Signed-off-by: Jean-Paul Saman <jpsaman@videolan.org>

Cherry-picked (with manual merge fix) from a6c8ae35.

Signed-off-by: Jean-Paul Saman <jpsaman@videolan.org>

This reverts commit c37341f1.
Wrong fix, do not trust Ubuntu users.

Fix [e6e22cae]

This reverts commit 55e7393c.
This need to be done in the GUIs

Really close #1227

This fixes #1578 for -bugfix. This breaks plugins from the source tree
(so we obviously need some more involved fix for master).