[CoreMedia] Improve CMFormatDescription.SubType properties. Fixes #23217.

[rolfbjarne]

Any CMPixelFormat value is also a valid CMVideoCodecType value, so duplicate
the values in CMPixelFormat into CMVideoCodecType.

This helps discovery/intellisense.

Also add a CMMediaSubType enum that mirrors the Swift type with the same name, which can be used to improve discovery/intellisense as well.

Additionally, fix a few other issues as well.

Fixes #23217.

[jeremy-visionaid]

I'd expect the macios projection of CMVideoCodecType to include only those values from CMVideoCodecType. So, adding pixel formats to it looks incorrect to me if they don't match the upstream definitions.

The reason for the initial bug report is that I'd also expect there to be some projection of the values of MediaSubType so that it's possible to write code that's a closer match to the Apple API. e.g.

AVCaptureDeviceFormat desc = <something>;
if (desc.MediaSubType == MediaSubType.PixelFormat_32BGRA)
...

I think it's more difficult to find things in the dotnet API the further they are from the upstream definition. So, this PR is kind of the opposite of what I was intending to achieve in opening #23217.

As Rolf pointed out, all the constants from MediaSubType are actually available in other types (despite the difficultly in actually reasoning that that's the case). Just adding an extra convenience property would be more helpful IMHO:

 public CVPixelFormatType PixelFormatType { 
 	get { 
 		return MediaType == CMMediaType.Video ? (CVPixelFormatType) MediaSubType : 0; 
 	} 
 } 

However, the trouble is with it being a video mediatype is that you don't know whether it is compressed or not, so either the existing VideoCodecType property or PixelFormatType above might cast to an undefined enum value. (i.e. IMHO it would be better to just have the MediaSubType constants than any of the convenience properties).

Obsoleting CAC3 I think is a good change though, since I'd expect to find it from the usptream identifier iec60958AC3 rather than named from its FourCC value. 👍

[jeremy-visionaid]

LGTM now. Nice one, thanks!

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