Send modifiers even without AddFb2
When running headless, only the invalid modifiers are advertised.
That breaks with the NVIDIA proprietary driver which then rejects the buffers created with the invalid modifier, and that kills Xwayland, meaning that running Xwayland on top of a mutter based compositor headless is not possible.
The reason the modifiers are not sent is because AddFb2 is not supported when running headless.
Other compositors (weston, wlroots) would still send the modifiers even without AddFb2, and Xwayland works find on those compositors when running headless.
Remove the requirement for AddFb2 to send the modifiers, so that Xwayland can work fine on top of mutter headless with the NVIDIA proprietary driver.
Closes: #3060 (closed)