libplacebo: fix dependency linkage

[rcombs]

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All of libplacebo's dependencies are technically optional, but some of them are essential to calling packages (e.g. mpv). In particular, much of the library's functionality depends on linking against either shaderc or glslang, and being compiled against vulkan headers, but the build won't fail by default in the absence of either.

Currently, we're failing to pull in any of these dependencies in CI builds, for a few different reasons, all of which are resolved here:

• We were missing a build-time dependency on pkg-config; added one
• We attempted to link against glslang, but meson was unable to find it; fixed by switching to shaderc, which provides a .pc file that meson can find (shaderc is also preferred by libplacebo; it only uses glslang as a wrapper around it)
• We didn't pass -D[dep]=enabled args, so failures to link deps were ignored; added them
• We had a dependency on sdl2, but it's been unused for a while, as libplacebo only uses SDL for the plplay test utility, which is only built if depending on ffmpeg, which was removed in libplacebo: remove ffmpeg dependency #132349 (that removal is probably still reasonable, as the ffmpeg package will probably later want to depend on libplacebo for functionality more useful to most people than a test program)

This fixes the usage of libplacebo in mpv (the only current dependent package), which was almost entirely broken previously, at least in CI builds.

[iMichka]

Looks great. Would it be possible to squash the 4 commits into one and force push to your branch?

[rcombs]

@iMichka done.

[github-actions]

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