win-dshow: Fix crashing when using a custom vcam placeholder

The scaler assumed the placeholder was the same size as the camera which
caused crashes if the user replaced the placeholder with a smaller
resolution image (or if the camera was potentially running at > 1080p).
This adds a separate scaler for the placeholder and uses the resolution
of the virtual camera instead of defaulting to 1080p.
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f