Update bundled libwebp to version 1.0.0

This commit imports libwebp 1.0.0, including AUTHORS, COPYING, ChangeLog,
NEWS, PATENTS, README and src directories. In src, only includes header
and source files.

Upstream changes since 0.6.1 have been merged in.
Also updated version in qt_attribution.json.

[ChangeLog][Third-Party Code] Update bundled libwebp to version 1.0.0.

Change-Id: Ia30ccc90286d5dd3e48e091f101f1cae84785150
Reviewed-by: Kai Koehne <kai.koehne@qt.io>
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>