Clang build warning

When building with Clang I get this warning.

```
# github.com/chai2010/webp
In file included from z_libwebp_src_enc_picture_csp_enc.c:9:
../../../go/pkg/mod/github.com/chai2010/webp@v1.1.1/internal/libwebp-1.0.2/src/enc/picture_csp_enc.c:1002:40: warning: result of '2 ^ ALPHA_OFFSET' is 1; did you mean '1 << ALPHA_OFFSET' (8)? [-Wxor-used-as-pow]
../../../go/pkg/mod/github.com/chai2010/webp@v1.1.1/internal/libwebp-1.0.2/src/enc/picture_csp_enc.c:1002:40: note: replace expression with '0x2 ^ ALPHA_OFFSET' to silence this warning
```

clang version 10.0.0-4ubuntu1
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

Reproduce a Clang build of github.com/chai2010/webp and inspect internal/libwebp-1.0.2/src/enc/picture_csp_enc.c around line 1002, where the warning is reported. Done means the build no longer emits this warning without introducing a regression.

Written by the indexing model from the issue text.