v4l2src: preserve features when fixating caps

The caps features were lost when sorting caps structures in
gst_v4l2src_fixate(). This was breaking alternate as
GST_CAPS_FEATURE_FORMAT_INTERLACED was removed from the caps.
13 jobs for <span class="ref-name">v4l2-caps</span> in 36 minutes and 23 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Preparation
passed #216653
gst indent

00:00:30

passed #216652
manifest

00:00:45

 
  Build
passed #216654
build fedora x86_64

00:05:09

passed #216655
build nodebug fedora x86_64

00:05:05

 
  Test
passed #216656
check fedora

00:03:09

passed #216657
integration testsuites fedora

00:06:13

 
  Full Builds
passed #216659
build cerbero android universal

00:11:02

passed #216660
build cerbero cross win32

00:07:24

passed #216661
build cerbero cross win64

00:04:14

passed #216658
build cerbero fedora x86_64

00:05:40

 
  Full Tests
passed #216663
valgrind bad

00:05:34

passed #216662
valgrind good

00:07:02

 
  Apps
passed #216664
android universal examples

00:06:10