debug: add new element fakevideodec

The fake video decoder ignores input bitstream except to enforce caps restrictions. It reads video width, height and framerate from caps. Then it just pushes video frames without doing any decoding.

The fake video decoder just draws a snake moving from left to right in the middle of the frame. This is a light weight drawing while it still provides an idea about how smooth is the rendering.

The fake video decoder inherits from GstVideoDecoder. It is useful to measure how smooth will be the whole rendering pipeline if you had the most efficient video decoder. Also useful to bisect issues for example when suspecting issues in a specific video decoder.

Handles mpeg2, mpeg4, h263, h264, theora, vp8, wmv3, msmpeg, flash-video, vp6, vp9, wmv1, wmv2, divx but more can be added if needed.

For now it can only output RGBA, RGBx, BGRA, BGRx.

Its rank is 0 (none) but I added a property to change it so that it can be selected by decodebin.

gst-launch-1.0 fakevideodec rank=512
playbin uri=http://clips.vorwaerts-gmbh.de/big_buck_bunny.mp4

http://bugzilla.gnome.org/show_bug.cgi?id=723778

Closes #679