Swap OptiPNG with OxiPNG

This makes building simpler and allows us to potentially use multithreading version in the future.

For now points to a custom fork of OxiPNG that enables WebAssembly support, as PR is still pending review.

codecs/oxipng/src/lib.rs

@@ -0,0 +1,7 @@
+use wasm_bindgen::prelude::*;
+
+#[wasm_bindgen(catch)]
+pub fn optimise(data: &[u8], level: u8) -> Vec<u8> {
+  let options = oxipng::Options::from_preset(level);
+  oxipng::optimize_from_memory(data, &options).unwrap_throw()
+}