Suggestion for CMake builds: formats relying on external dependencies off by default.

[MatheusKS95]

I make a suggestion: support for certain file formats that require the dependencies on "external" dir (such as AVIF) should be set off by default, instead of on. Thus, newcomers and people who for any reason forget to set such flags off won't get tripped by build errors (and most people use formats such as PNG anyways, and SDL_image don't need external libs to load PNGs).

It's a "QOL" kind of issue/suggestion, not a "OMG everything is breaking call 911" issue, however. But myself and a lot of other people (per SDL's Discord) tripped on this.

tl;dr everything that relies on what's inside "external" dir should be off by default instead of on.\

edit: removed personal tale and added a tl;dr;