ColorPicker picking color on Linux is unusable when single window mode is disabled

[WhalesState]

Tested versions

4.4.dev7

System information

Linux

Issue description

• This was not implemented on Linux, makes it impossible to pick a color from screen on linux, 9fece03

Steps to reproduce

Pick a color from screen on linux.

Minimal reproduction project (MRP)

N/A

[Calinou]

I can't reproduce this on 4.4.dev aa65940 (Linux X11 KDE, single window mode enabled in the Editor Settings):

color_picker_linux_x11_single_window_mode.mp4

However, if you have single window mode disabled (the default), it will constantly flicker back and forth:

Epilepsy warning (flashing lights)

color_picker_linux_x11_multi_window.mp4

This is spammed to stderr while moving the cursor:

ERROR: Invalid image: null
   at: create_from_image (scene/resources/image_texture.cpp:76)

This issue didn't occur in 4.3 and is likely a regression from #88950. cc @bruvzg