thorvg: Disable webp loader if corresponding module is disabled

[akien-mga]

Fixes #91873.

[akien-mga]

This should fix the logic, but further work seems needed (in a follow up).

From a quick test, I can't get SVG with embedded <image> bitmaps to be loaded in Godot, whether I use PNG, JPG or WebP.

They also don't show up in the ThorVG web viewer which seems to outright fail loading those SVGs.

Edit: It seems to work if the images are linked, but not embedded (base64). Judging by the ThorVG code there's no support implemented for embedded images? Need further investigation.

[hermet]

@akien-mga hello, do you have a issued svg to test? thorvg basically supports the embedded images.

[fire]

I have posted a image sample to thorvg.