macOS: Support retina display (#96)

Apply angband/angband#416 changes to support retina on macOS.

src/main-cocoa.m

@@ -1587,19 +1587,33 @@ - (void)updateImage
 
     CGLayerRelease(self.angbandLayer);
 
+    /* Use the highest monitor scale factor on the system to work out what
+     * scale to draw at - not the recommended method, but works where we
+     * can't easily get the monitor the current draw is occurring on. */
+    float angbandLayerScale = 1.0;
+    if ([[NSScreen mainScreen] respondsToSelector:@selector(backingScaleFactor)]) {
+        for (NSScreen *screen in [NSScreen screens]) {
+            angbandLayerScale = fmax(angbandLayerScale, [screen backingScaleFactor]);
+        }
+    }
+
     /* Make a bitmap context as an example for our layer */
     CGColorSpaceRef cs = CGColorSpaceCreateDeviceRGB();
     CGContextRef exampleCtx = CGBitmapContextCreate(NULL, 1, 1, 8 /* bits per component */, 48 /* bytesPerRow */, cs, kCGImageAlphaNoneSkipFirst | kCGBitmapByteOrder32Host);
     CGColorSpaceRelease(cs);
 
     /* Create the layer at the appropriate size */
-    size.width = fmax(1, ceil(size.width));
-    size.height = fmax(1, ceil(size.height));
+    size.width = fmax(1, ceil(size.width * angbandLayerScale));
+    size.height = fmax(1, ceil(size.height * angbandLayerScale));
     self.angbandLayer =
         CGLayerCreateWithContext(exampleCtx, *(CGSize *)&size, NULL);
 
     CFRelease(exampleCtx);
 
+    /* Set the new context of the layer to draw at the correct scale */
+    CGContextRef ctx = CGLayerGetContext(self.angbandLayer);
+    CGContextScaleCTM(ctx, angbandLayerScale, angbandLayerScale);
+
     [self lockFocus];
     [[NSColor blackColor] set];
     NSRectFill((NSRect){NSZeroPoint, [self baseSize]});