don't use opengl for all windows

[Antoine Martin]

The point of using opengl for rendering windows is the performance: we take advantage of the GPU for painting YUV directly, we can handle repaints very quickly.

For many types of windows, this is not particularly useful: tooltips, menus don't last long enough do benefit. There can be many small windows, we don't need to register an opengl contect for every one of them.
And in some cases it can actually be detrimental (crashes: #2669).

[Antoine Martin]

Done in r26044.

[Antoine Martin]

See also: #2735 (skip using opengl for 'text' windows)

[migration script]

this ticket has been moved to: ​https://github.com/Xpra-org/xpra/issues/2721