[google_maps_flutter_ios] Fix kCGImageAlphaPremultipliedLast implicit conversion from enumeration type (#9720)

[Docs for `CGBitmapContextCreate ` `bitmapInfo`](https://developer.apple.com/documentation/coregraphics/cgcontext/init(data:width:height:bitspercomponent:bytesperrow:space:bitmapinfo:)?language=objc) parameter say:
> The constants for specifying the alpha channel information are declared with the [CGImageAlphaInfo](https://developer.apple.com/documentation/coregraphics/cgimagealphainfo?language=objc) type but can be passed to this parameter safely. You can also pass the other constants associated with the [CGBitmapInfo](https://developer.apple.com/documentation/coregraphics/cgbitmapinfo?language=objc) type.

The docs still [show `kCGImageAlphaPremultipliedLast` being passed in directly](https://developer.apple.com/library/archive/documentation/GraphicsImaging/Conceptual/drawingwithquartz2d/dq_context/dq_context.html#//apple_ref/doc/uid/TP30001066-CH203-CJBHBFFE).

Instead of casting to `CGBitmapInfo` (which would also work I believe), use [`kCGBitmapAlphaInfoMask`](https://developer.apple.com/documentation/coregraphics/cgbitmapinfo/kcgbitmapalphainfomask).

Further explanation at inkling/Subliminal#23 (comment).

Tested here:
https://github.com/flutter/packages/blob/b2aef15c15a75ac6853dc1b79ee2ad2086934f58/packages/google_maps_flutter/google_maps_flutter_ios/example/ios14/ios/RunnerTests/GoogleMapsTests.m#L81

Fixes flutter/flutter#170440

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Author: jmagman

packages/google_maps_flutter/google_maps_flutter_ios/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 2.15.5
+
+* Fixes `kCGImageAlphaPremultipliedLast` implicit conversion from enumeration type warning.
+
 ## 2.15.4
 
 * Deprecates `zIndex` parameter in Marker in favor of `zIndexInt`.

packages/google_maps_flutter/google_maps_flutter_ios/example/ios14/ios/RunnerTests/GoogleMapsTests.m

@@ -84,7 +84,8 @@ - (void)testHandleResultTileDownsamplesWideGamutImages {
   size_t bitsPerComponent = CGImageGetBitsPerComponent(imageRef);
 
   // non wide gamut images use 8 bit format
-  XCTAssert(bitsPerComponent == 8);
+  XCTAssertEqual(bitsPerComponent, 8);
+  XCTAssertEqual(CGImageGetAlphaInfo(imageRef), kCGImageAlphaPremultipliedLast);
 }
 
 - (void)testAnimateCameraWithUpdate {

packages/google_maps_flutter/google_maps_flutter_ios/ios/Classes/FLTGoogleMapTileOverlayController.m

@@ -96,8 +96,9 @@ - (UIImage *)handleResultTile:(nullable UIImage *)tile {
   // If it is wide gamut, we want to downsample it
   if (isFloat & (bitsPerComponent == 16)) {
     CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
-    CGContextRef context = CGBitmapContextCreate(nil, tile.size.width, tile.size.height, 8, 0,
-                                                 colorSpace, kCGImageAlphaPremultipliedLast);
+    CGContextRef context =
+        CGBitmapContextCreate(nil, tile.size.width, tile.size.height, 8, 0, colorSpace,
+                              (kCGBitmapAlphaInfoMask & kCGImageAlphaPremultipliedLast));
     CGContextDrawImage(context, CGRectMake(0, 0, tile.size.width, tile.size.height), tile.CGImage);
     CGImageRef image = CGBitmapContextCreateImage(context);
     tile = [UIImage imageWithCGImage:image];

packages/google_maps_flutter/google_maps_flutter_ios/pubspec.yaml

@@ -2,7 +2,7 @@ name: google_maps_flutter_ios
 description: iOS implementation of the google_maps_flutter plugin.
 repository: https://github.com/flutter/packages/tree/main/packages/google_maps_flutter/google_maps_flutter_ios
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+maps%22
-version: 2.15.4
+version: 2.15.5
 
 environment:
   sdk: ^3.6.0