`sdl3_image`: add `SUPPORT_SAVE_WEBP` support for Android (SDL_image 3.4.0+)

SDL_image 3.4.0 added `IMG_SaveWEBP`, but building with `SUPPORT_SAVE_WEBP=true` fails with linker errors (`WebPAnimEncoderNewInternal`, `WebPMuxCreateInternal`, etc.) because `libwebpmux` and `libwebpenc` are not compiled or linked.

**Current workaround:** `SUPPORT_SAVE_WEBP := false` (matches older SDL_image releases).
