[CoreGraphics] Adjust CGPDFOperatorTable to not use a generic Action delegate.

[rolfbjarne]

• CoreCLR doesn't support generic Action delegates in reverse (P/Invokes), so
  we need to find a different solution.
• The native CGPDFOperatorTable callback API is not very friendly to us,
  because we can't pass it callback-specific data, which means that the
  managed caller must conform to the FullAOT requirement of the managed
  callback (must be a static function; must have a MonoPInvokeCallback
  attribute).
• We can leverage the new function pointer syntax in C# 9 to make these
  requirements enforced by the C# compiler (unmanaged function pointer +
  UnmanagedCallersOnly attribute). The resulting API is still clunky to use,
  but I don't see any way around that.

Fixes this monotouch-test failure with CoreCLR:

[FAIL] Tamarin : System.Runtime.InteropServices.MarshalDirectiveException : Cannot marshal 'parameter #3': Non-blittable generic types cannot be marshaled.
           at CoreGraphics.CGPDFOperatorTable.CGPDFOperatorTableSetCallback(IntPtr table, String name, Action`2 callback)
           at CoreGraphics.CGPDFOperatorTable.SetCallback(String name, Action`2 callback)
           at MonoTouchFixtures.CoreGraphics.PDFScannerTest.Tamarin() in xamarin-macios/tests/monotouch-test/CoreGraphics/PDFScannerTest.cs:line 102

Ref: dotnet/runtime#32963

[spouliot]

👍

[tj-devel709]

Woah I don't recall seeing this before haha. A type cast for null! What does this do?

[mandel-macaque]

@tj-devel709 imaging you are a compiler and you see:

public void CGPDFOperatorTableSetCallback (IntPtr handel, String name, CGPDFOperatorCallback callback) {...}
public void CGPDFOperatorTableSetCallback (IntPtr handel, String name, OtherObject obj) {..}

Later in the code I have:

CGPDFOperatorTableSetCallback (Handle, name, null);

Which of the above overloads do you call??

And if you see:

CGPDFOperatorTableSetCallback (Handle, name, (CGPDFOperatorCallback) null);

AFAIK you can also do:

CGPDFOperatorTableSetCallback (Handle, name, default(CGPDFOperatorCallback));

which helps the compiler too without a cast.

[tj-devel709]

Ah thank you for explaining!

[vs-mobiletools-engineering-service2]

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