video-blend: fix blending of rectangles partially or fully outside of the video

video-blend: fix blending of rectangles partially or fully outside of the video

In case of overlay being completely or partially outside
the video frame, the offset calculations are not right,
which resulted in the overlay not being displayed as
expected, or crashes due to invalid memory access.

When the overlay rectangle is completely outside,
we need not render the overlay at all.

For partial display of overlay rectangles, src_yoff
was not being calculated, hence it was always clipping
the bottom half of the overlay, By calculating the
src_yoff, now the overlay is clipped properly.

https://bugzilla.gnome.org/show_bug.cgi?id=739281