macos to handle more clipboard formats, converting them on the fly

[totaam]

At the moment, we simply drop these types of clipboard data:

if type in ("WINDOW", "PIXMAP", "BITMAP", "DRAWABLE", "PIXEL", "COLORMAP"):
    debug("skipping clipboard data of type: %s, format=%s, len(data)=%s", dtype, dformat, len(data))
    return None, None

We could try to handle some of those, and provide them in multiple formats since we generally have PIL available for converting between formats.

From a security POV, it probably makes sense to always convert formats so that we can "guarantee" that the data we send over the wire is not malicious?
Think: an application providing a JPEG based buffer overflow via the clipboard: worst case scenario is that the xpra server crashes parsing it or maybe it gets compromised, but the client machine will not receive the malicious content directly.
But then again, if you can exploit the server, you can then inject the bad content in there.. I guess it's still a first line of defense.

[totaam]

Now that both OSX and win32 are using synchronous clipboard code (pretty much) and OSX is using at least some native call (see #318 for details)
It probably makes sense to use native libraries directly for accessing rich formats:

• [http://docs.activestate.com/activepython/2.4/pywin32/win32clipboard.html] win32clipboard (simple example here)
• AppKit NSPasteboard

[totaam]

Some links:

• gtk-osx pasting images from clipboard
• Get Pasted Image from Clipboard Firefox

See also #2289

[totaam]

For win32: #2619

[totaam]

html5 was done in #2312

This only leaves macos for this ticket. (summary updated)

Examples:

• pypi : pasteboard
• AppKit example - maybe using NSTIFFPboardType ?
• example code:

def copyToPasteboard_(self, string):
        pasteboard = NSPasteboard.generalPasteboard()
        pasteboard.declareTypes_owner_([NSStringPboardType], self)
        pasteboard.setString_forType_(string, NSStringPboardType)

[totaam]

MacOS clipboard updates:

• r26670 implements images support, for other data types: macos: handle more clipboard data types #2807.
• r26672 + r26678 + r26679 + r26685: debug logging improvements
• r26683 + r26688 handle more formats, honour "want-targets"
• r26684 use pillow if we need to convert the image
• r26689 fix sending images to the macos clipboard

Generic clipboard updates:

• r26680: simplify owner-changed handler
• r26681: don't wait to send lightweight clipboard tokens
• r26682: peers that request 'want-targets' need to be notified of owner changes as the targets are very likely to have changed with it

We should also sanitize images in both directions: #2808.