Add a WebP decoder

Container and lossy decoding by Aneesh Dogra <aneesh@sugarlabs.org>
Lossless decoding by Justin Ruggles <justin.ruggles@gmail.com>

libavcodec/allcodecs.c

@@ -253,6 +253,7 @@ void avcodec_register_all(void)
     REGISTER_DECODER(VP6F,              vp6f);
     REGISTER_DECODER(VP8,               vp8);
     REGISTER_DECODER(VQA,               vqa);
+    REGISTER_DECODER(WEBP,              webp);
     REGISTER_ENCDEC (WMV1,              wmv1);
     REGISTER_ENCDEC (WMV2,              wmv2);
     REGISTER_DECODER(WMV3,              wmv3);