* if the given vout thread is a filter, then don't sleep untill the display
date. This reduces the chance of pictures being late when reaching the vout
if one or more filters are in use
 * don't include current render time in the render_time sliding mean if it
is way too large ( as inspired by Simon Gittins ).
 * weight the current sliding mean tree times more than the current render
time when calculating the new render time. This should make the code more
robust to jitter in render time