#995 Soft subtitle canvas dimensions are limited to video canvas dimensions

[Alex Folland]

To reproduce:

1. Play a video with SRT subtitles using the default subtitle style settings.
2. Expand the viewport's height to significantly more than the video canvas' height.
3. Watch until a subtitle line is rendered.

Observed behavior:

The soft subtitle canvas dimensions are limited to the video canvas dimensions, even when the viewport is vertically larger than the video. This forces subtitles which are intended to be anchored to the bottom of the subtitle canvas to entirely overlap the video canvas even when there is ample viewport space below the video canvas.

Expected behavior:

The soft subtitle canvas dimensions are expected to match the viewport dimensions, regardless of the video dimensions, to prevent entirely overlapping the video canvas when the viewport has ample excess height beyond the video canvas' height.