Warning in picture_csp_enc.c related to XOR operation

**Description**:
When building my Go Fyne project which uses the `github.com/chai2010/webp` library, I encountered the following warning:

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.

**Steps to reproduce**:
1. Running the app or even building it with go build .
2. fyne package and then run the executable

**Environment**:
- Go version: 1.20.1
- OS: MacOS 13.5.1

Start at internal/libwebp-1.0.2/src/enc/picture_csp_enc.c around line 1002 and inspect the XOR expression that triggers the compiler warning. Reproduce it with go build . or the reported Fyne packaging flow, then verify the warning is gone without changing WebP encoding behavior.

Written by the indexing model from the issue text.