chore: draw the toolbar icons in-tree with make icons (closes #378)

script/lib/icons.js draws the mark from geometry read back out of the
committed PNGs, since the coordinates were never recorded, and writes each
PNG with node's own zlib. `make icons` runs it and leaves alone a file that
already holds the drawn image.

tests/icons.test.js requires each committed file to hold exactly that
image: the same IHDR and every pixel. The compressed bytes are not
compared, because node's bundled zlib does not compress as the stock zlib
that made the committed files did; the decision is recorded on the issue.

build.js copies the manifests from MANIFEST_SOURCES instead of naming the
two paths a second time.

Model: opus-5-5

tests/icons.test.js

@@ -0,0 +1,33 @@
+// The toolbar icons in icons/ are drawn by script/lib/icons.js (`make icons`).
+// Each committed file must hold exactly the image it draws, the same IHDR and
+// every pixel, so the drawing and the files cannot drift apart. The compressed
+// bytes are not compared: the committed files were compressed by stock zlib,
+// and node's bundled zlib compresses the same pixels differently.
+
+const fs = require("fs");
+const path = require("path");
+
+const { icons } = require("../manifest/chrome.json");
+const { drawIcon, decodePng } = require("../script/lib/icons");
+
+describe("toolbar icons", () => {
+    test.each(Object.entries(icons))(
+        "the %spx icon %s holds the image script/lib/icons.js draws",
+        (size, file) => {
+            const side = Number(size);
+            const committed = decodePng(
+                fs.readFileSync(path.join(__dirname, "..", file)),
+            );
+            const drawn = decodePng(drawIcon(side));
+
+            expect(committed.header).toEqual(drawn.header);
+            // Each row is its filter type byte, then 4 bytes per pixel.
+            expect(drawn.rows.length).toBe(side * (side * 4 + 1));
+            expect(committed.rows.length).toBe(drawn.rows.length);
+            // The offset of the first byte that differs, not a diff of all.
+            expect(
+                committed.rows.findIndex((byte, i) => byte !== drawn.rows[i]),
+            ).toBe(-1);
+        },
+    );
+});