refresh video_frame again, but now only if needed (no cpu usage anymore, if not needed)

[kdenlive] / effects / grain.xml

<!DOCTYPE kpartgui>
<effect tag="grain">
        <name>Grain</name>
        <description>Noise on Picture</description>
        <author>Marco Gittler</author>
        <properties id="grain" tag="grain" />
        <parameter type="constant" name="noise" max="255" min="0" default="40">
                <name>Noise</name>
        </parameter>
</effect>