artemp/issue 978

[artemp]

Improve Image and ImageView objects scopes and lifetime management.

• This PR adds Ref() / Unref() logic to Image::encode and ImageView::encode async methods.
  Under load input Image /ImageView objects can get GC-ed while AsyncEncode::Execute methods hasn't completed. To avoid this, we increment reference count in sync part or the encode and decrement in async handler when op is complete ( AsyncEncode::OnWorkComplete(Napi::Env env, napi_status status) )

• Add persistent Buffer reference to Image to ensure underlying buffer stays in scope (in situations when Image doesn't own underlyling data)

• Image - update ctor and remove ad-hoc _buffer field

• Use Napi::EscapableHandleScope in Image::buffer and Image::data

[springmeyer]

Overall this looks like a great improvement since it fixes a critical issue that (I think?) regressed. I've added some comments inline below. Could you also add a general description to the PR that:

• describes what was broken (and why)
• describes why this now works right