faster client startup

[Antoine Martin]

Same as #2341 but for the client.

We already have:

• #2344 for codecs
• #2345 for opengl probing

[Antoine Martin]

Added some debug logging in r23053 + r23054 + r23055.

Places where we spend most of the startup time:

• opengl=probe takes 1s on my system - can easily take more with other GPUs / drivers, hence the need for #2345
• import xpra.platform.gui takes 120ms (not sure why yet)
• import xpra.client.UIXpraClient takes ~40ms (not bad considering how much that imports)
• get_platform_info takes 40ms - this could be done from a separate thread
• importing the picture and video codecs: 70ms - could be done in parallel with connecting to the server
• opengl re-tests despite the initial probe (only once now: r23056) - difficult to fix
• setup_menu: 120ms (mostly loading icons from disk - not much we can do? is it worth using pillow to load from a separate thread?)

Start to finish only takes 2 seconds, more than half of which is the opengl probing!
With --no-speaker --no-microphone --opengl=no --encodings=rgb --video-decoders=none --csc-modules=none --tray=no, my python3 client starts in just 0.5s.

[Antoine Martin]

Updates:

• r23062 makes it possible to skip loading the menu icons
• r23063 debug logging reveals that there is a delay whilst loading the xpra.png icon (importing pillow, etc) - can't really avoid that
• 30ms delay before get_vrefresh in ​https://github.com/mviereck/x11docker/files/3345452/xpraclient.log
• r23067 faster linux distro indentification
• r23068 delay loading of X11 and GTK bindings

That's enough I think.

[Antoine Martin]

See also: #2349 cache audio query