Currently Windows gets this from vcpkg. Mac will get it from Homebrew,
and the GitHub linux builders will just install it with apt-get.

This might not be the perfect solution (having to install Homebrew is a pain,
GitHub Actions has an ancient SDL2), but it gets all the common platforms
running for now without much fuss.

This doesn't fix the audio gaps, just the static introduced in the SDL2 port.

SDL2 does not initialize the audio callback's buffer, unlike SDL 1.2, under
the assumption the callback is going to fully write it anyhow. But since
the movie player wants to mix against the current contents of the buffer,
we need to explicitly initialize it to silence first.

Fixes keyboard input during gameplay.

…000.

Update SDL 1.2 references, remove SDL_image references.