fakevideosink: request an extra buffer if enable-last-sample is enabled

If the 'enable-last-sample' property is enabled, fakevideosink will keep
a reference on last rendered buffer which may lead to buffer starvation
in the pipeline.
Request one extra buffer in this case so we always have a buffer flying
in the pipeline.

https://bugzilla.gnome.org/show_bug.cgi?id=795109
1 file changed