[Impeller] made window size parametric in playground/golden tests (flutter/engine#46682)

In https://github.com/flutter/flutter/issues/136058#issuecomment-1751507094 it was seen that some bugs manifest only with viewports of certain sizes.  This allows writers of tests to adjust the window size for playgrounds and golden image tests.

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

engine/src/flutter/impeller/golden_tests/golden_playground_test.h

@@ -57,6 +57,9 @@ class GoldenPlaygroundTest
 
   ISize GetWindowSize() const;
 
+ protected:
+  void SetWindowSize(ISize size);
+
  private:
 #if FML_OS_MACOSX
   // This must be placed first so that the autorelease pool is not destroyed

engine/src/flutter/impeller/golden_tests/golden_playground_test_mac.cc

@@ -194,4 +194,8 @@ ISize GoldenPlaygroundTest::GetWindowSize() const {
   return pimpl_->window_size;
 }
 
+void GoldenPlaygroundTest::GoldenPlaygroundTest::SetWindowSize(ISize size) {
+  pimpl_->window_size = size;
+}
+
 }  // namespace impeller

engine/src/flutter/impeller/golden_tests/golden_playground_test_stub.cc

@@ -64,4 +64,6 @@ ISize GoldenPlaygroundTest::GetWindowSize() const {
   return ISize();
 }
 
+void GoldenPlaygroundTest::SetWindowSize(ISize size) {}
+
 }  // namespace impeller

engine/src/flutter/impeller/playground/playground.h

@@ -92,6 +92,8 @@ class Playground {
 
   virtual bool ShouldKeepRendering() const;
 
+  void SetWindowSize(ISize size);
+
  private:
   struct GLFWInitializer;
 
@@ -105,8 +107,6 @@ class Playground {
 
   void SetCursorPosition(Point pos);
 
-  void SetWindowSize(ISize size);
-
   FML_DISALLOW_COPY_AND_ASSIGN(Playground);
 };