RFR: 8160597: IllegalArgumentException when we initiate drag on Image

[Jose Pereda]

This PR fixes an exception that can happen when dragging an image from a WebView on macOS.

Dragging an image that is directly included like `<img src="some image"/>` works fine, without exception. However, there are (at least) two other cases when the IAE is thrown: 

- Dragging an image that is part of an hyperlink, like `<a href="some url"><img src="some image"/></a>`.
- Dragging an image encoded as base64.

The IAE happens only on macOS, where a native image is created in `GlassPasteboard.m` from an URL, and in those two cases the generated image has 0x0 dimensions:
 
`<NSImage 0x... Size={0, 0} ...>`

which leads to `ByteArrayFromPixels` being called with a null byte array. 

The included test reproduces the case by directly pushing the `MacPasteboard` content (instead of adding a more complex test with a webView and a drag and drop gesture).

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Commit messages:
 - Remove unused imports
 - Check if MacPasteboard contains a valid image, including test

Changes: https://git.openjdk.java.net/jfx/pull/668/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=668&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8160597
  Stats: 155 lines in 5 files changed: 154 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/668.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/668/head:pull/668

PR: https://git.openjdk.java.net/jfx/pull/668