Use UNALIGNED_ACCESS_IS_FAST for OxiPNG -> libdeflate

I only observed ~10-15% increase in perf on single-threaded bench, which isn't much, but no harm in adding flag anyway.

Fixes #833.

codecs/oxipng/build.sh

@@ -3,6 +3,7 @@
 set -e
 
 rm -rf pkg,{-parallel}
+export CFLAGS="${CFLAGS} -DUNALIGNED_ACCESS_IS_FAST=1"
 wasm-pack build -t web
 RUSTFLAGS='-C target-feature=+atomics,+bulk-memory' wasm-pack build -t web -d pkg-parallel -- -Z build-std=panic_abort,std --features=parallel
 # Workaround https://github.com/rustwasm/wasm-bindgen/issues/2133: