Add JPEG-XL, AVIF support

[Orochimarufan]

Both are available starting with SDL2_image 2.6.0, but must be explicitly enabled at build-time.

Also let python query successful initialization of specific formats. I'm unsure if this was originally intended, but pygame_sdl2.image.init() will only fail if ALL requested formats fail to initialize. This gives renpy the opportunity to check if any actually used formats are required. I'm open to suggestions as to better ways of handling missing formats in SDL2_image. Unfortunately, it is rather cumbersome to query support of formats that don't have to be initialized, so this was omitted (they are enabled in the default configuation anyway).

Fixes #136