Update libwebp to 0.6.0-pre

Corresponds with Android change
https://android-review.googlesource.com/#/c/326439/

"this prerelease snapshot includes encoder and performance
improvements"

Update build file
* Many files have been renamed from
    src/<subdir>/<name>.c
      to
    src/<subdir>/<name>_<subdir>.c

* Build new files (*_msa.c, *_neon.c, predictor_enc.c)

This should fix issue 5876, which was caused by a compiler bug.
With the added NEON implementation, we will no longer trigger
the bug.

BUG=skia:5876

Change-Id: I0fcce4362ee70138547b1d23aa9ef537a4126e73
Reviewed-on: https://skia-review.googlesource.com/7376
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
2 files changed