libobs: Make sure all code paths leave context

[suogesi]

Description

Not calling gs_leave_context after gs_enter_context can cause a lock leak. This PR makes sure all code paths in obs_init_video_mix from libobs/obs.c will do this.

Motivation and Context

#13366

This is NOT a complete fix of this issue. OBS should show error messages to remind users not to write such resolution because GPU cannot process it.

This PR makes "Stop Recording" work but the output is not as expected. (i.e., if you write "1920x1079" into Settings > Output (Advanced) > Recording > Rescale Output, you won't get a 1920x1079 video. Instead, you get a default resolution.)

How Has This Been Tested?

1. Select "NVIDIA NVENC H.264" at Settings > Output (Advanced) > Recording > Video Encoder.
2. Select "Bilinear" at Settings > Output (Advanced) > Recording > Rescale Output.
3. Write "1920x1079" into Settings > Output (Advanced) > Recording > Rescale Output.
4. Click "Start Recording".
5. Click "Stop Recording".
6. Video saved instead of keeping "Stopping Recording...".

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have read the contributing document.
• My code has been run through clang-format.
• My code follows the project's style guidelines
• My code is not on the master branch.
• My code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[Warchamp7]

This will need to be properly rebased and squashed instead of the merge commit.

[PatTheMav]

Almost a poster boy example for the benefits of the defer statement, which would allow us to write:

defer {
    gs_leave_context();
}

if (video-gpu_conversion && !obs_init_gpu_conversion(video)) {
    return OBS_VIDEO_FAIL;
}

if (!obs_init_textures(video)) {
    return OBS_VIDEO_FAIL;
}

return OBS_VIDEO_SUCCESS;

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Until we can make use of that fabled C language version, we have to use the change as suggested by the PR.