Replace zlib, zstd, libchdr, libzip and libpng with libretro-common

Deflate, zlib and gzip streams go through rinflate and rdeflate, zstd
through rzstd, CHD images through rchd, zip archives through
rzip_archive and PNG through rpng. The bundled libraries, their build
glue and the system-library options are gone; adler32 and uncompress
keep zlib's ABI for the prebuilt FFmpeg archives that still call them.
libretro-common is one CMake library that the core, the headless tool
and the unit tests all link.

The reads take shorter paths on the way. CHD decodes straight from the
loader's reads and a range of blocks at a time, and a file read at any
alignment decodes into place; CSO keeps one inflate stream instead of
one per block; a plain ISO serves a file read with one read of its exact
bytes; a stored zip member is read in place and a deflated one through
restart points rather than a copy of the whole image in memory; zip
members decode into the caller's buffer; sequential reads through the
frontend's VFS skip the seek, which also keeps its buffering.

libretro/test reads one image as ISO, CSO, CHD and zip through the block
devices, boots each, and checks the codecs against Python-made vectors;
a CI job runs it.

Core/FileLoaders/LocalFileLoader.cpp

@@ -187,8 +187,18 @@ size_t LocalFileLoader::ReadAt(s64 absolutePos, size_t bytes, size_t count, void
 
 #if defined(HAVE_LIBRETRO_VFS)
 	std::lock_guard<std::mutex> guard(readLock_);
-	File::Fseek(file_, absolutePos, SEEK_SET);
-	return fread(data, bytes, count, file_);
+	// The VFS has no positioned read. Only seek when the read isn't sequential:
+	// a seek also throws away whatever the frontend's stream had buffered.
+	if (filePos_ != absolutePos) {
+		if (File::Fseek(file_, absolutePos, SEEK_SET) != 0) {
+			filePos_ = -1;
+			return 0;
+		}
+		filePos_ = absolutePos;
+	}
+	const size_t done = fread(data, 1, bytes * count, file_);
+	filePos_ = done == bytes * count ? filePos_ + (s64)done : -1;
+	return done / bytes;
 #elif PPSSPP_PLATFORM(SWITCH)
 	// Toolchain has no fancy IO API.  We must lock.
 	std::lock_guard<std::mutex> guard(readLock_);