Draft: compositor: add state transactions

• Closes #3787
• Supersedes !4501

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Rough overview:

• A state group is a group of cached states which must be committed at the same time
• A transaction is a short-lived set of cached states
  • Constructed by merging multiple state locks, right before committing
  • Committed alongside an optional pending state
    • An optimization to allow applying the pending state directly
  • Committing is optimized to avoid creating a state group when possible
    • An empty transaction
    • A transaction with only one state
    • A transaction which can be applied immediately
  • Uses an external array of surface pointers to avoid excessive reallocations

TODO:

• Transaction API documentation
• transaction_commit() documentation
• More testing

Breaking changes

• wlr_surface_lock_pending() and wlr_surface_unlock_cached() have been removed. Instead, a new state lock API is introduced.

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randfall branch for debugging: https://gitlab.freedesktop.org/vyivel/randfall/-/tree/dbg-txn