presentation-time: Specify refresh bounds for VRR

When this extension was developed, we did not yet know how VRR hardware
would behave in practice as it was not standardised, and the KMS
interface was equally unstandardised.

Now things have shaken out to an acceptable level, we have a good idea
of what we need, which is simply to include a base refresh rate - the
rate the compositor would drive the display for non-VRR clients.

Bump the protocol to version 2 and require the compositor to provide
a rate.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Derek Foreman <derek.foreman@collabora.com>