Add WebP support to AppImage

[s09bQ5]

At least one Person (@barbeque-squared) uses WebP images with USDX. The DLLs for the Windows build support WebP, the AppImage for Linux currently does not.

• Extend dl.sh and tasks.sh to download and build libwebp.
• Configure SDL2_image to use that library.

[s09bQ5]

I just noticed that FFmpeg contains decoders for many image formats (jpeg, png, webp, tiff, bmp, pnm, pcx, tga, jpeg 2000, xpm, xbm, ...). Maybe we should try to use these if SDL2_image fails or has been disabled at build time. Images are treated as videos with a single frame.

[barbeque-squared]

That's an interesting idea. Is FFmpeg something that USDX will always need? And if so, would it make sense to try and see if we can drop the SDL2_image dependency altogether? I don't know if/how this will work with stuff like transparency in menu icons, or maybe there's performance considerations to prefer SDL2_image.
(now watch as people start (ab?)using this to set mp3's with embedded coverart as the #COVER or something...)