Create an overlay that will display the bones of the active armature as an overlay.
The bones of the selected armature will not be drawn by the object mode draw engine

To achieve the fade out we render to an framebuffer what is initialized with color (0, 0, 0, 0.5) And use DRW_STATE_BLEND. Z-depth should also be cleared.

We will reuse the drawing shaders of object mode.

Add an operation connected to the Z-Key that enables/disables this overlay.

note: we currently do not implement the local bone selection as it is not supported yet by the operations.
note: the overlay is only available when pose mode is active. hence we implement it as part of the pose mode.
note: Should we implement a MULTISAMPLE_SYNC_ENABLE/DISABLE that can work towards a custom framebuffer?