Use default visual, depth and colormap in systray embedder window

We were using the Emacs' frame's depth, but not the visual nor colormap.
This failed with Emacs 29 and its support for 32-bit depths.

We now use the default screen's visual: using a non-default visual in
the system tray requires support for embedding icons with different
visuals, which is not implemented.  We restrict our limited transparency
support to Emacs frames with depth equal to the default visual's detph.

* exwm-core.el (exwm--get-visual-depth-colormap): New function.
* exwm-systemtray.el (exwm-systemtray--init): Use root window's
visual, depth and colormap. Reset all attributes that refer
(perhaps due to defaults) to the parent window, as it might have a
different visual, depth or colormap.
(exwm-systemtray--init): Set _NET_SYSTEM_TRAY_VISUAL.
(exwm-systemtray-background-color): Emit a warning when
transparency is selected but not supported.
(exwm-systemtray--set-background-color): New function to set
embedder window background.
(exwm-systemtray--embedder-window-depth): Add variable.
(exwm-systemtray--transparency-supported-p): New function to check
whether transparency is supported.

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