Reland "[Impeller] Fail if software backend is chosen and Impeller is enabled on iOS." (flutter#46275)

Reverts flutter/engine#46217

cl/568091248 makes this safe to land again.

fyi @jonahwilliams @chinmaygarde

Author: dnfield

shell/platform/darwin/ios/framework/Source/FlutterEnginePlatformViewTest.mm

@@ -73,7 +73,9 @@ - (void)setUp {
                                /*io=*/thread_task_runner);
   platform_view = std::make_unique<flutter::PlatformViewIOS>(
       /*delegate=*/fake_delegate,
-      /*rendering_api=*/flutter::IOSRenderingAPI::kSoftware,
+      /*rendering_api=*/fake_delegate.settings_.enable_impeller
+          ? flutter::IOSRenderingAPI::kMetal
+          : flutter::IOSRenderingAPI::kSoftware,
       /*platform_views_controller=*/nil,
       /*task_runners=*/runners,
       /*worker_task_runner=*/nil,
@@ -91,8 +93,13 @@ - (void)tearDown {
 }
 
 - (void)testMsaaSampleCount {
-  // Default should be 1.
-  XCTAssertEqual(platform_view->GetIosContext()->GetMsaaSampleCount(), MsaaSampleCount::kNone);
+  if (fake_delegate.settings_.enable_impeller) {
+    // Default should be 4 for Impeller.
+    XCTAssertEqual(platform_view->GetIosContext()->GetMsaaSampleCount(), MsaaSampleCount::kFour);
+  } else {
+    // Default should be 1 for Skia.
+    XCTAssertEqual(platform_view->GetIosContext()->GetMsaaSampleCount(), MsaaSampleCount::kNone);
+  }
 
   // Verify the platform view creates a new context with updated msaa_samples.
   // Need to use Metal, since this is ignored for Software/GL.