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

tools/make_icon.m

@@ -1,6 +1,6 @@
-// Builds the 1024x1024 iOS app icon from the game's own 128 px icon (read from the local install).
-// The gear is scaled with high-quality interpolation onto an opaque dark background
-// (iOS app icons must be opaque).
+// Builds the 1024x1024 iOS app icon from the project logo (assets/logo.png, transparent).
+// The emblem is drawn with high-quality interpolation onto an opaque, full-bleed dark background:
+// iOS app icons must be opaque and square (the system applies the rounded mask).
 // Usage: make_icon SOURCE_PNG OUTPUT_PNG
 #import <Foundation/Foundation.h>
 #import <CoreGraphics/CoreGraphics.h>
@@ -21,7 +21,7 @@ int main(int argc, char **argv) {
         CGContextDrawRadialGradient(ctx, gradient, CGPointMake(size / 2.0, size * 0.55), 0,
                                     CGPointMake(size / 2.0, size / 2.0), size * 0.75, kCGGradientDrawsAfterEndLocation);
         CGContextSetInterpolationQuality(ctx, kCGInterpolationHigh);
-        CGFloat inset = size * 0.12;
+        CGFloat inset = size * 0.14;  // keep the emblem clear of the mask's corners
         CGContextDrawImage(ctx, CGRectMake(inset, inset, size - 2 * inset, size - 2 * inset), gear);
         CGImageRef icon = CGBitmapContextCreateImage(ctx);
         CGImageDestinationRef dst = CGImageDestinationCreateWithURL((__bridge CFURLRef)[NSURL fileURLWithPath:@(argv[2])],