wlr-screencopy: wait for buffer_done for buffer negotiation

[russelltg]

Fixes #127

Could you test & lmk how it goes?

Good catch--doesn't seem like I was particularly careful about this one.

Curious, for a new compositor why wlr-sreencopy and not ext-image-copy-capture? It's not like loads of stuff support the wlr- one anyways and ext-image-copy-capture is quite nice.

[kenoss]

Negotiation looks working correctly and the app failed as my compostior returns Xbgr8888 first. Thanks!

$ cargo run
   Compiling wl-screenrec v0.1.7 (/home/kenoss/src/github.com/russelltg/wl-screenrec)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 1.58s
     Running `target/debug/wl-screenrec`
08:25:06 [WARN] hw flag is specified, but there's no known vaapi codec for MPEG4. Using a generic encoder...
[AVHWDeviceContext @ 0xaaaafdde2240] Failed to initialise VAAPI connection: -1 (unknown libva error).
08:25:06 [ERROR] failed to create encoder(s): Failed to load vaapi device: Input/output error. This is likely *not* a bug in wl-screenrec, but an issue with your vaapi installation. Follow your distribution's instructions. If you're pretty sure you've done this correctly, create a new issue with the output of `vainfo` and if `wf-recorder -c h264_vaapi -d /dev/dri/renderD128` works.

Stack backtrace:
(...snip...)

---

Curious, for a new compositor why wlr-sreencopy and not ext-image-copy-capture? It's not like loads of stuff support the wlr- one anyways and ext-image-copy-capture is quite nice.

Long story short,

1. Ecosystem is not yet matured.
   1-1. There is no compositor-agnostic XDG Portal backend using ext-image-copy-capture like xdg-desktop-portal-wlr for wlr-screencopy yet. COSMIC has its backend, but IIUC it's for COSMIC.
   1-2. grim doesn't support ext-image-copy-capture yet.
2. wlr-screencopy is relatively easier to implement.
3. Smithay looks to have a plan [smithay#781] to implement ext-image-copy-capture-v1.

So, I decided to implement wlr-screencopy for a shot-term solution.