Set thumb image as the attribute poster of video automatically

[dallaslu]

ref #3428

[rhukster]

Can you please provide a use case for this PR in this "conversation" section of the PR itself. I read your issue, but it's not clear which use case/example this PR is addressing.

[dallaslu]

Example:

Auto poster

Both 1280x720_1mb.mp4 and 1280x720_1mb.mp4.thumb.jpg was uploaded to /user/pages/02.typography/.

/user/pages/02.typography/default.md:

![Video Sample](1280x720_1mb.mp4)

/user/themes/{theme}/templates/default.html.twig:

{{ page.media['1280x720_1mb.mp4'].html()|raw }}

Both will get the same HTML:

<video poster="/user/pages/02.typography/1280x720_1mb.mp4.thumb.jpg" alt="">
	<source src="/user/pages/02.typography/1280x720_1mb.mp4">
	Your browser does not support the video tag.
</video>

Disable poster

When 1280x720_1mb.mp4.thumb.jpg is missing, the poster attribute is not output by default.

Or manually set it to not output poster.

/user/pages/02.typography/default.md:

![Video Sample](1280x720_1mb.mp4?poster=0)

/user/themes/{theme}/templates/default.html.twig:

{{ page.media['1280x720_1mb.mp4'].poster(0).html()|raw }}

HTML Result:

<video alt="">
	<source src="/user/pages/02.typography/1280x720_1mb.mp4">
	Your browser does not support the video tag.
</video>

Manual

The manual assignment of the poster function is not affected.

[kniziol]

It might be simplified in this way:

Suggested change

	if (!isset($attributes['poster']) || ($attributes['poster'] !== 0 && $attributes['poster'] !== '0')) {
	$this->setPoster($attributes);
	$this->removePoster($attributes);

To make it work, you need to create the following methods in the VideoMediaTrait trait:

    private function isPosterUnknown(array $attributes): bool
    {
        return !isset($attributes['poster']) || (int) $attributes['poster'] !== 0;
    }

    private function removePoster(array $attributes): void
    {
        if ($this->isPosterUnknown($attributes)) {
            return;
        }

        unset($attributes['poster']);
    }

    private function setPoster(array $attributes): void
    {
        if ($this->isPosterUnknown($attributes) && $this->thumbnailExists('page')) {
            $thumb = $this->get('thumbnails.page', false);

            if ($thumb) {
                $thumb = $thumb instanceof ThumbnailImageMedium
                    ? $thumb
                    : MediumFactory::fromFile($thumb, ['type' => 'thumbnail']);

                $attributes['poster'] = $thumb->url();
            }
        }
    }

The names of new methods may be different. I want to show you the idea. Of course, you'll need to double-check that and ensure it works as expected.