jpegenc produces data in subsampling mode that rtpjpegpay can't handle

Submitted by Olivier Crête @ocrete

Link to original bug (#684502)

Description

We probably need to add more data to the image/jpeg caps to specify it's subsampling mode so it can be configured, but I don't really understand how jpegenc works.

Test case:

gst-launch-1.0 videotestsrc ! 'video/x-raw, width=(int)160, height=(int)120, framerate=(fraction)30/1, format=(string)BGRx, pixel-aspect-ratio=(fraction)1/1' ! jpegenc ! rtpjpegpay ! fakesink

Setting pipeline to PAUSED ...
Pipeline is PREROLLING ...
ERROR: from element /GstPipeline:pipeline0/GstRtpJPEGPay:rtpjpegpay0: Invalid component
Additional debug info:
gstrtpjpegpay.c(544): gst_rtp_jpeg_pay_read_sof (): /GstPipeline:pipeline0/GstRtpJPEGPay:rtpjpegpay0
ERROR: pipeline doesn't want to preroll.