Expose device list update func

[bazile-clyde]

Description

This PR exports the logic executed at startup to find and register cameras. This will allow us to discover cameras connected after our application has started.

[at-wat]

Do you think we can expose setup function in other drivers as well? It would be better to have unified interface to update device list.

[codecov[bot]]

Codecov Report

Base: 58.16% // Head: 58.21% // Increases project coverage by +0.04% :tada:

Coverage data is based on head (b4118d2) compared to base (6ac1424). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #463 +/- ## ========================================== + Coverage 58.16% 58.21% +0.04% ========================================== Files 62 62 Lines 3679 3683 +4 ========================================== + Hits 2140 2144 +4 Misses 1412 1412 Partials 127 127 ``` | [Impacted Files](https://codecov.io/gh/pion/mediadevices/pull/463?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | Coverage Δ | | |---|---|---| | [pkg/driver/camera/camera\_linux.go](https://codecov.io/gh/pion/mediadevices/pull/463?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion#diff-cGtnL2RyaXZlci9jYW1lcmEvY2FtZXJhX2xpbnV4Lmdv) | `30.43% <100.00%> (+0.76%)` | :arrow_up: | | [pkg/driver/screen/x11\_linux.go](https://codecov.io/gh/pion/mediadevices/pull/463?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion#diff-cGtnL2RyaXZlci9zY3JlZW4veDExX2xpbnV4Lmdv) | `12.69% <100.00%> (+2.86%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion)

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[bazile-clyde]

Do you think we can expose setup function in other drivers as well? It would be better to have unified interface to update device list.

@at-wat Done! I skipped the dummy.go files in audiotest and videotest. I also skipped vncdriver.go since there's no func init. Let me know if I should not have skipped any of those.

[at-wat]

@bazile-clyde thanks!

I think it would be better in future to have another API that mediadevices.MediaDevices can automatically call them to update the device list. For this, Initialize functions would be better to have godoc comments to mention this API is experimental. What do you think?

[bazile-clyde]

@bazile-clyde thanks!

I think it would be better in future to have another API that mediadevices.MediaDevices can automatically call them to update the device list. For this, Initialize functions would be better to have godoc comments to mention this API is experimental. What do you think?

That makes sense. I don't have strong opinions either way so I added it. Thanks for the quick turn-around btw!

[bazile-clyde]

Could you revert import block orders to keep them like

import (
  "stdlib1"
  "stdlib2"

  "nonstdlib1"
  "nonstdlib2"
)

Done!