Separate SDLimage, SDLttf and SDLmixer into separate subsystems

[nsrahmad]

That way, If I don't want any of them and only depend on the main SDL3 apis only, I don't need to load them. Note that loading them correctly also requires additional dll's, which can be avoided if they are in separate subsystem.

[atomontage]

Also consider further conditionalizing trivial-main-thread, it's bringing in additional dependencies and not all of us care about it on Darwin, as we can 'trivially' create our own solutions.

Also consider dropping defclass-std, reasoning being that a foundational wrapper library whose aim is to be used by as many people as possible should strive for a minimal set of dependencies.

[aiffc]

ok let me do these later