Previously, `Frame` was stored in KMS frame udata, but in some cases the
udata was dropped without a capture happening, and `Frame` did not
implement `Drop`, so `fail` was never sent.
Instead, rename `DropableFrame` to `Frame` and `Frame` to `FrameRef`, so
we can have a single instance of `Frame`, that will send `fail` on drop.
This guarantees either `.success` or `.fail` are send, as long as its
not leaked.
This seems to fix https://github.com/pop-os/cosmic-comp/issues/1305.
xdg-desktop-portal-cosmic prints an error, buy retries (as it should for
an `Unknown` error; though maybe there should be a retry limit) and the
session continues working.
(Not sure if it should be sending `failed`, or queing it with the next
frame so it can send `success` to the client, but this works and is
desirable as a failsafe anyway.)
`Session` and `CursorSession` are similiarly updated.
`.fail()`, `.success()`, and `.stop()` now consume
`Frame`/`Session`/`CursorSession`. So to stop a session, it is now
necessary to call `.remove_session()`, but then simply dropping with
send `.stop()`.
Factoring out some `Request::Capture` handling into a `capture_frame`
function seems to clean up error handling and such a bit.
