kmssink: Do not source using padded width/height

The width/height from the video meta can be padded width, height. But when sourcing from padded buffer, we only want to use the valid pixels. This rectangle is from the crop meta, orther it is deduces from the caps. The width and height from the caps is save in the parent class, use these instead of the GstVideoInfo when settting the src rectangle.

This fixes an issue with 1080p video displaying repeated or green at the padded bottom 8 lines (seen with v4l2codecs).

cc @vjaquez