Issue #8110: Add "Use Sharp YUV" option to webp export

Attempts to resolve the issue noted in #8110 (closed). This adds a new "Advanced Option" frame to the webp export dialogue where users can choose to enable "Sharp YUV" for conversion. The feature already exists in libwebp; it just has to be enabled with webp_config.use_sharp_yuv = 1; in the export code.

The conversion produced by GIMP is identical to this example image: https://www.ctrl.blog/media/video/webp-sharp-yuv-vs-webp.apng

The proposed UI for the dialogue looks like this: