Prepare Skia-Webp decoder for Alpha and lossless.

Change the webp_parse_header to read appropriate header bytes (required
for Alpha & lossless bit-stream);
Replaced call WebPGetInfo with WebPGetFeatures to get hasAlpha
information;
Replaced hard-coded setIsOpaque(true) with setIsOpaque(!hasAlpha);
Refactored code for setting decoder config;

Change-Id: I8208233d1aaa0a213a35dd996a72e43f78901c89
1 file changed