stbt.py: reduce memory used when queue is non-leaky

By allocating the queue after the ffmpgecolorspace element, we reduce the
memory consumed when it is non-leaky, that is in a `process_all_frames`
context.

To reduce this even more, I've tried to substitute this queue with a queue2
element. However, what queue2 does is store the buffers in disk AS WELL AS
keep them in memory. Therefore, that doesn't help.

The amount of memory used also depends on the processing speed of the
computer. Currently, on a machine with an Intel Core i5, Fedora 18, and
hardly any process running besides the default services of Fedora, this
consumes an extra ~200MB every second.
stbt.py: process_all_frames must ensure there is incoming video

Due to the fact the `process_all_frames()` creates the Queue with unlimited
size from scratch, makes it possible that a test fails due to NoVideo
just because the first thing it does is a call to `process_all_frames()`.

This commit adds this extra step where the queue must have at least one
element before the yield statement.