omxh26xenc: Negotiate subframe mode

We now negotiate subframe mode through the caps. To enabled subframe
mode, the caps need to specify alignment=nal:

  ... ! omxh264enc ! video/x-h264,alignment=nal ! ...
  ... ! omxh265enc ! video/x-h265,alignment=nal ! ...
8 jobs for video_encoder_subframe_support in 15 minutes and 5 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #711229
gst indent

00:00:18

passed #711228
manifest

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  Build
passed #711234
build clang fedora x86_64

00:04:15

passed #711230
build fedora x86_64

00:09:40

passed #711231
build nodebug fedora x86_64

00:08:46

passed #711232
build static fedora x86_64

00:12:04

passed #711233
build static nodebug fedora x86_64

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  Integrate
passed #711235
documentation

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