contrib: add libplacebo

Use a pinned git commit and build statically due to libplacebo's
unstable alpha API. The chosen hash corresponds to the tag v0.1.0.
Signed-off-by: Thomas Guillem <thomas@gllm.fr>

contrib/src/libplacebo/SHA512SUMS

+fad25b4c429aecbd446a5d890a293a7f21267ea4d68f0cbbf9c0e1d97a742b2229d640c0a90c1290decc24d684d74cce186a45c31d821f797b69147c3f7005fb  libplacebo-0.1.0.tar.gz

contrib/src/libplacebo/rules.mak

+# libplacebo
+
+PLACEBO_VERSION := 0.1.0
+PLACEBO_URL := https://github.com/haasn/libplacebo/archive/v$(PLACEBO_VERSION).tar.gz
+PLACEBO_ARCHIVE = libplacebo-$(PLACEBO_VERSION).tar.gz
+
+PLACEBOCONF := --prefix="$(PREFIX)" \
+	--libdir lib \
+	--default-library static
+
+$(TARBALLS)/$(PLACEBO_ARCHIVE):
+	$(call download_pkg,$(PLACEBO_URL),libplacebo)
+
+.sum-libplacebo: $(PLACEBO_ARCHIVE)
+
+libplacebo: $(PLACEBO_ARCHIVE) .sum-libplacebo
+	$(UNPACK)
+	$(MOVE)
+
+.libplacebo: libplacebo
+	cd $< && rm -rf ./build
+	cd $< && $(HOSTVARS) meson $(PLACEBOCONF) build
+	cd $< && cd build && ninja install
+	touch $@