Add project logo and use it for the app icon

- assets/logo.png (transparent) and assets/logo-256.png for the site
  and README
- tools/make_icon.m builds the full-bleed iOS icon from the logo

docs/building.md

@@ -66,7 +66,7 @@ only.
 2. **Retarget.** `tools/retarget.py` writes `build/native/Factorio.dylib`, changing only Mach-O
    header bytes, and records the changes in `build/native/retarget.json`.
 3. **Shims and host.** The shim libraries and the small host executable are compiled for iOS
-   with clang; the app icon is generated from the game's own icon.
+   with clang; the app icon is generated from `assets/logo.png`.
 4. **Bundle and sign.** `build/native/Factorio.app` is assembled, signed, and installed with
    `devicectl`.
 5. **Data.** `--push-data` copies the game's `data/` folder (~1.5 GB) into the app's