Many video encoders fail to negotiate the source format

Submitted by Francois Gouget

Link to original bug (#756004)

Description

It looks like more video encoders have a similar source format negotiation problem to bug 750398:

To reproduce the problem simply run:
$ GST_DEBUG=1 gst-launch-1.0 videotestsrc num-buffers=40 ! videoconvert ! avenc_apng ! fakesink
Setting pipeline to PAUSED ...
Pipeline is PREROLLING ...
ERROR: from element /GstPipeline:pipeline0/GstVideoTestSrc:videotestsrc0: Internal data flow error.
Additional debug info:
gstbasesrc.c(2943): gst_base_src_loop (): /GstPipeline:pipeline0/GstVideoTestSrc:videotestsrc0:
streaming task paused, reason not-negotiated (-4)
ERROR: pipeline doesn't want to preroll.
Setting pipeline to NULL ...
Freeing pipeline ...

Here is a list of the video encoders that seem to be broken:
avenc_alias_pix
avenc_apng
avenc_avrp
avenc_avui
avenc_ayuv
avenc_dpx
avenc_flashsv2
avenc_jpeg2000
avenc_jpegls
avenc_pbm
avenc_r10k
avenc_snow
avenc_sunrast
avenc_utvideo
avenc_v308
avenc_v408
avenc_v410
avenc_xbm
avenc_xface
avenc_xwd
avenc_y41p
avenc_yuv4

In each case it looks like videoconvert should be able to convert the source image format into the format needed by the encoder.

Version: 1.6.0