[Impeller] fail if software backend is chosen and Impeller is enabled…

… on iOS (#46124)

Fixes flutter/flutter#127408

This is also related to #44346, which made it fatal to explicitly request both impeller and the software backend.

Before landing this, we need to update some google internal tests that end up in this state to explicitly request Skia (or to get into a mode where they can actually use metal).

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

@@ -72,7 +72,9 @@ - (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,
@@ -90,8 +92,13 @@ - (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.