vfb: new mode not in XRRSizes of current connection

[Antoine Martin]

Tested using the example in r27697.

Running the same command again sees the new resolution:

$ DISPLAY=:10 python3 ./tests/xpra/x11/add_vfb_size.py 1600 1200
2020-10-19 12:25:10,717 get_xrr_screen_sizes()=[(5760, 2560), (3840, 2160), (100, 100), (1920, 1080), (1024, 768), (800, 600)]
screen_sizes=[(5760, 2560), (3840, 2160), (100, 100), (1920, 1080), (1024, 768), (800, 600)]
2020-10-19 12:25:10,717 add_screen_size(1600, 1200)
2020-10-19 12:25:10,717 adding mode 0x2ae to output 0x3d
add_screen_size(1600, 1200)=(1600, 1200)
2020-10-19 12:25:11,718 get_xrr_screen_sizes()=[(5760, 2560), (3840, 2160), (100, 100), (1920, 1080), (1024, 768), (800, 600)]

$ DISPLAY=:10 python3 ./tests/xpra/x11/add_vfb_size.py 1600 1200
screen_sizes=[(5760, 2560), (3840, 2160), (100, 100), (1920, 1080), (1024, 768), (800, 600), (1600, 1200)]
resolution (1600, 1200) is already present!

But if we wait for XRRSizes to return it, it never does!
Something must be getting cached somewhere in the X11 server?

[Antoine Martin]

Workaround applied in r27698.

[migration script]

this ticket has been moved to: ​https://github.com/Xpra-org/xpra/issues/2910