[macOS] Fix missing frame_changed signal to CameraFeed

[shiena]

fixed #104808
I will add the missing emit of the frame_changed signal in the CameraFeed class on macOS.

• Production edit: Related to Fix missing frame_changed signal to CameraFeed godot-sdk-integrations/godot-ios-plugins#78.

[shiena]

The frame_changed signal is documented but not implemented. Because this signal doesn't arrive, the datatype, width, and height remain unknown, preventing decoding and aspect ratio determination.

https://docs.godotengine.org/en/stable/classes/class_camerafeed.html#class-camerafeed-signal-frame-changed

[shiena]

When updating frames in camera feed, we always call one of set_rgb_image / set_ycbcr_image / set_ycbcr_images / set_external. Therefore, we're moving the signal emission from platform-specific CameraFeedXXX classes to the base CameraFeed class.

[shiena]

Currently, the CameraFeed::frame_changed signal is essential due to the following non-functional scenarios:

• CPU-based image processing libraries like OpenCV
• Frame change detection when CameraFeed is reactivated after being deactivated (i.e., active → inactive → active cycle)

[BastiaanOlij]

I think from a high level this makes sense. I don't really have the ability to test this.

@stuartcarnie is this maybe something you can verify just to make sure?

My only coding remark is that it's better to create a constant for the StringName itself, have a look at other places in the code where we do this. It's more efficient than constructing new StringNames all over. But that really is a nitpick.

[shiena]

@BastiaanOlij
Changed constants to StringName type.