v4l2object: improve colorspace handling for JPEG sources

v4l2object: improve colorspace handling for JPEG sources

gstjpegdec sets 1:4:0:0 colorimetry (full range BT.601 YCbCr encoding
with unknown primaries and unknown transfer function). This currently
gets translated to bt601 or bt709 depending on resolution.
Both cases result in a negotiation failure:

    ERROR: from element /GstPipeline:pipeline0/v4l2video0convert:v4l2video0convert0: Device '/dev/video0' does not support 1:4:0:0 colorimetry

Improve the guessing game by selecting JPEG colorimetry (JPEG colorspace
with sRGB transfer function) under these specific conditions, and loosen
the matching so that 1:4:0:0 input gets accepted if the device is
actually configured to 1:4:7:1 (V4L2_PIX_FMT_JPEG default).

https://bugzilla.gnome.org/show_bug.cgi?id=796940