[FEATURE] A SKPixmap from a disposed SKSurface should throw an exception on .GetPixels call

[The-MAZZTer]

Is your feature request related to a problem? Please describe.

When learning Skia I did not realize the Surface was required to be live and undisposed to use the Pixmap. So I ended up with an AccessViolationException sometimes when using .GetPixels after disposing the Surface via a using block.

Describe the solution you'd like

It would be nice if the SKPixmap class could detect this condition and throw an exception instead of returning a pointer to freed memory.

Describe alternatives you've considered

Alternatively the Surface could wait until all Pixmaps created from it are disposed before disposing itself. This is more complex than necessary, though, and violates the contract of IDisposable.

Additional context
I am using SkiaSharp in Unity where the AccessViolationException causes Unity Editor to hard crash. Not fun!

Additional API calls on Pixmap besides GetPixels should also throw the Exception. Possibly all of them.

VS bug #820075

[mattleibow]

This is a good thing to do. Thanks.

[The-MAZZTer]

As a side note ReadPixels returns TRUE for a successful copy, though it doesn't actually seem to copy anything (the destination buffer remains zeroed instead of having the expected image data in it).

Other possible alternate solutions I thought up since my original report, for your consideration:

1. Disposing the SKSurface could also Dispose the SKPixmaps created from it automatically. This could result in .GetPixels returning IntPtr.Zero or throwing ObjectDisposedException (the latter being the standard thing to do AFAIK). However again this is not usually what Dispose is expected to do.
2. Create a patch for Skia itself to implement behavior on the native side and submit it upstream. I don't think there's any Dispose-like pattern in use there, so having the get pixels API return a null pointer may be the solution there.

[The-MAZZTer]

If you need reproduction steps for your Triage you can Dispose the SKSurface and then call .GetPixels, and feed the IntPtr into Marshal.Copy or Texture2D.LoadRawImageData.

I have found a large image size (1000x1000) the AccessViolationException happens all the time, while a smaller 500x500 it only errors occasionally. However since the Dispose itself doesn't always result in the AccessViolationException. but only sometimes, it is probably some garbage collection on the C side or the GC on the .NET side (in which case forcing a garbage collection with the GC static methods after the Dispose should help reproduce it).