Downscale pasted PNG images based on metadata (#29123)

Some images like MacOS screenshots contain
[pHYs](http://www.libpng.org/pub/png/book/chapter11.html#png.ch11.div.8)
data which we can use to downscale uploaded images so they render in the
same dppx ratio in which they were taken.

Before:

<img width="584" alt="image"
src="50979e3a-5d5a-40dc-a0a4-36eb6e28f14a">

After:

<img width="329" alt="image"
src="0690902a-f2fe-4c6b-97b3-6fdd67c21bad">

(cherry picked from commit 5e72526da4e915791f03af056890e16821bde052)

web_src/js/utils/image.js

@@ -0,0 +1,47 @@
+export async function pngChunks(blob) {
+  const uint8arr = new Uint8Array(await blob.arrayBuffer());
+  const chunks = [];
+  if (uint8arr.length < 12) return chunks;
+  const view = new DataView(uint8arr.buffer);
+  if (view.getBigUint64(0) !== 9894494448401390090n) return chunks;
+
+  const decoder = new TextDecoder();
+  let index = 8;
+  while (index < uint8arr.length) {
+    const len = view.getUint32(index);
+    chunks.push({
+      name: decoder.decode(uint8arr.slice(index + 4, index + 8)),
+      data: uint8arr.slice(index + 8, index + 8 + len),
+    });
+    index += len + 12;
+  }
+
+  return chunks;
+}
+
+// decode a image and try to obtain width and dppx. If will never throw but instead
+// return default values.
+export async function imageInfo(blob) {
+  let width = 0; // 0 means no width could be determined
+  let dppx = 1; // 1 dot per pixel for non-HiDPI screens
+
+  if (blob.type === 'image/png') { // only png is supported currently
+    try {
+      for (const {name, data} of await pngChunks(blob)) {
+        const view = new DataView(data.buffer);
+        if (name === 'IHDR' && data?.length) {
+          // extract width from mandatory IHDR chunk
+          width = view.getUint32(0);
+        } else if (name === 'pHYs' && data?.length) {
+          // extract dppx from optional pHYs chunk, assuming pixels are square
+          const unit = view.getUint8(8);
+          if (unit === 1) {
+            dppx = Math.round(view.getUint32(0) / 39.3701) / 72; // meter to inch to dppx
+          }
+        }
+      }
+    } catch {}
+  }
+
+  return {width, dppx};
+}