Add progress hook granularity in lossless

A WebPPicture instance is necessary to call WebPReportProgress() which
sets WebPPicture::error_code so as well use WebPEncodingSetError() to
record errors too, instead of functions returning a WebPEncodingError.
However there must be one WebPPicture instance per thread, with error
codes merged at sync time. A mutex could simplify that but it is not
the objective of this change.

https://groups.google.com/a/webmproject.org/g/webp-discuss/c/yOiP8APubgc/m/vCTvxl6ODgAJ

Change-Id: Ia1a8f9d1199202e1c88484ce719b0180a80447ce

src/webp/encode.h

@@ -441,7 +441,7 @@ WEBP_EXTERN int WebPPictureCrop(WebPPicture* picture,
 // the original dimension will be lost). Picture 'dst' need not be initialized
 // with WebPPictureInit() if it is different from 'src', since its content will
 // be overwritten.
-// Returns false in case of memory allocation error or invalid parameters.
+// Returns false in case of invalid parameters.
 WEBP_EXTERN int WebPPictureView(const WebPPicture* src,
                                 int left, int top, int width, int height,
                                 WebPPicture* dst);