new fakevideodec element

Submitted by Julien Isorce @cap

Link to original bug (#723778)

Description

I have started a fakevideodec element here http://cgit.collabora.com/git/user/julien/gst-plugins-good.git/commit/?h=fakevideodec&id=7a4c1ce6e3ecaa35ffbd74b7e37968e3adc70485

It's useful when you have a new embedded platform and you want to know what would be the performance if you had a decoder that use 0% CPU. (hardware decoder except the zero-copy part)

videotestsrc is not really usable on embedded like RPI, I mean it uses so much CPU so not really useful to identify what would be the best FPS.

Also fpsdisplaysink uses textoverlay for the visual fps information. When you want a visual information only (and not a console info)

Also fakevideodec is compatible with playbin as long as you increase its rank.

For now fakevideodec just draw a kind of snake on 1 line (to make it use the CPU the less possible)
And the snake moves from left to right in 1 sec if no drop. So that when there are frame dropping the snake freez and you see it jumps to other positions. At every new frame it clears the line and draw the next position base on the framerate and the resolution.

I made it quite quickly so it may not being exactly correct right now.

Also even if it currently visually gives an idea of the FPS, it still does not allow to determine visually what is the precise FPS (can't say 24 or 25 FPS)

It could be improved to draw a kind of minimal meter and then draw a spot to indicate what is the current framerate. There are several ideas.

Do no hesitate to put a comment :)