meson: correctly install the headers
Using install_headers with subdirectories like that breaks completely. Solve it by just using subdir, which also liberates us from having to manually list all of the headers. Due to a weird limitation of subdir(), we're forced to rename our public include dir to `libplacebo`. So take this opportunity to also move it to `include/libplacebo`. This has the pretty significant advantage that you can just use -I to test directly from a local libplacebo tree, which is a net gain overall.
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