cmake: add SDL3IMAGE_STRICT option to switch between required 3rd party libraries or optional ones

[madebr]

A short summary is also printed at the end:

-- SDL3_image backends:
-- - enabled:  avif bmp gif jpg jxl lbm pcx png pnm qoi svg tga tif webp xcf xpm xv
-- - disabled: stb imageio wic

jpg means a jpeg backend is available: it can be libjpeg, stb, wic or imageio.

Fixes #413

[sezero]

With this patch in, I enabled webp and tif support in cmake file as of 0003e26

Also: Can we not do the same in SDL2_image and SDL2_mixer? It's annoying to have one option being disabled by default in cmake while it is enabled by default in autotools.