video-info: no api to access padded plane height

There is currently no API to easily access to the padded height of planes.

We can use gst_video_info_align() with top or bottom padding which will update the plane offsets (GST_VIDEO_INFO_COMP_OFFSET) but user has no way to easily find the actual height of each plane. It needs to be calculated using offsets and strides and so is format dependent.

Would it make sense to add something like GstVideoInfo.padded_height[GST_VIDEO_MAX_PLANES] and a macro GST_VIDEO_INFO_COMP_PADDED_HEIGHT similar as what we already have for strides?