transparent windows

[Antoine Martin]

Transparency aka "alpha channel" tracker ticket.

Original tickets and important changesets:

• #77 tray icons
• #262 disappearing tooltips
• #279 32-bit visuals and transparent windows
• #280 Systray icon transparency broken
• #385 opengl transparency (and more)
• #487 webp artifacts with transparency
• #500 small tooltip windows have transparency issues
• #577 detect lack of transparency support and disable it to save bandwidth
• #892 transparency forwarding is broken
• #961 transparency support is broken with pixmap backing
• #1145 pixbuf backend does not support transparency with desktop scaling enabled

[Antoine Martin]

Fixes in r16262:

• skip compositing check with opengl backend on macos (though transparency still doesn't work..)
• enable transparency by default on macos as this seems to work with both cairo and pixmap backends
• remove override in pixmap backing, use the superclass default (ie: enabled on macos)
• add better error logging when initializing / enabling transparency, especially failures

This reverts an earlier changeset: r4934 "transparency does not work on OSX" - as it works with cairo and pixmap, the opengl case may be fixable.
If not, we should disable it in the backend - which will save some bandwidth.