backends/native: Track CRTC support for signaling VRR

We can only update VRR state when the CRTC supports the VRR_ENABLED property. At the same time, an output should only be considered VRR capable if both the sink supports it (connector VRR_CAPABLE) and all possible CRTCs can signal VRR (VRR_ENABLED on all compatible CRTCs).

Fixes: fee33299 ("onscreen/native: Allow requesting frame synchronization")

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Closes #3337 (closed)