2.4 html5 updates

[Antoine Martin]

Follow up from #1670.

The changelog: log/xpra/trunk/src/html5

Lots of goodies in the latest chrome builds: ​Chrome 66 Beta: CSS Typed Object Model, Async Clipboard API, AudioWorklet:

• ​ImageBitmapRenderingContext: This new rendering context uses transfer semantics to take ownership of the pixels of an ImageBitmap? object, thus avoiding memory duplication and rasterization overhead.

(from ​The ImageBitmap rendering context for canvas) - not sure about zerocopy since this populates a canvas but worth a look

• ​Unblocking Clipboard Access: The new asynchronous clipboard API provides a promise-based means of reading from and writing to the clipboard. and As with many new APIs, navigator.clipboard is only supported for pages served over HTTPS. To help prevent abuse, clipboard access is only allowed when a page is the active tab. Pages in active tabs can write to the clipboard without requesting permission, but reading from the clipboard always requires permission.
• ​AudioWorklet: The AudioWorklet? object provides a new synchronous JavaScript? execution context which allows developers to programmatically control audio without additional latency and higher stability in the output audio. - looks like exactly what we need (see #1775)

[Antoine Martin]

See also #1847: html5 native notifications api

• r19293: menu popup over windows
• r19294 + r19295 + r19301 + r19318 + r19321: fix + update + tune broadway decoder (#1463)
• r19303: focus fix
• r19310 + r19391: video / media source debug fix
• r19317: mpeg1 #1816
• r19361: eos handling #1850
• r19538: open urls #1862
• r19570: fullscreen toggle

[Antoine Martin]

We should also implement the same code as r19727: skip sending keyboard and mouse if the server doesn't handle those, see ticket:1838#comment:8.

[Antoine Martin]

Updates:

• r19576: don't use dummy icon (avoids http errors in server log)
• r19578: disable webgl rendering (#1816)
• r19591 + r19624: cap video size (#1867)
• r19626, r19633, r19634, r20578: minor refactoring or fixups
• r19627: free data early
• r19629 + r19631: skip redraw when the document is hidden
• r19837 + r19846: path handling fix
• r19903, r20340: keyboard fixes (#1901)
• r19990: honour start-child
• r20523: notification layout fix
• r20524: show URL open requests in a notifications bubble
• r20532: fix remote logging string encoding (must use utf8)
• r20547: video is meant to default to "on" now
• r20574: sort countries alphabetically
• r20602: better protocol error logging
• fixes to server code to avoid breaking bencoder: r20601, r20597 (and many more)
• better bencoder error logging: r20599, r20598
• IE compatibility: #1973
• many keyboard fixes: #1898
• html toolbar improvements: #1817

[Antoine Martin]

Will follow up in #1979

[migration script]

this ticket has been moved to: ​https://github.com/Xpra-org/xpra/issues/1788