videodecoder: drop frame to be called on last subframe

As some decoders don't know in advance the number of subframes,
they'll receive, the decoder subclass should call drop_frame
only on whole frame or last subframe, to avoid breaking the logic of
one (VideoCodec)frame for a set of subframe buffers.

Indeed the decoder subclass should know when the decoding stage is
over for a given frame and should call either finish_frame or
drop_frame on the last subframe. See openjpegdec for a proper
example.

Add an API to get the current subframe number received by the
decoder subclass.
Update tests to check this API.
34 jobs for !454 with video_decoder_subframe_support in 12 minutes and 57 seconds (queued for 5 minutes and 55 seconds)
detached
Status Job ID Name Coverage
  Build Docker
passed #5754137
alpine amd64 manifest builder docker

00:05:32

passed #5754136
android docker

00:05:33

passed #5754135
cerbero fedora amd64 docker

00:05:47

passed #5754134
fedora amd64 docker

00:05:51

passed #5754138
gst-indent amd64 docker

00:05:51

 
  Preparation
passed #5754140
gst indent

00:05:32

manual #5754139
manual
manifest
 
  Build
created #5754149
gstreamer
build cerbero cross-android universal
created #5754153
gst-ios-13.2
build cerbero cross-ios universal
created #5754150
gstreamer
build cerbero cross win32
created #5754151
gstreamer
build cerbero cross win64
created #5754148
gstreamer
build cerbero fedora x86_64
created #5754152
gst-macos-10.15
build cerbero macos x86_64
created #5754154
1809 gstreamer-windows windows docker
build cerbero msvc x86_64
created #5754144
build clang fedora x86_64
created #5754147
1809 windows docker allowed to fail manual
build msys2
created #5754141
build nodebug fedora x86_64
created #5754142
build static fedora x86_64
created #5754143
build static nodebug fedora x86_64
created #5754145
1809 windows docker
build vs2017 amd64
created #5754146
1809 windows docker
build vs2017 x86
 
  Test
created #5754155
gstreamer
check fedora
created #5754156
gstreamer
integration testsuites fedora 1/4
created #5754157
gstreamer
integration testsuites fedora 2/4
created #5754158
gstreamer
integration testsuites fedora 3/4
created #5754159
gstreamer
integration testsuites fedora 4/4
created #5754163
gstreamer
valgrind bad
created #5754160
gstreamer
valgrind base
created #5754164
gstreamer
valgrind ges
created #5754161
gstreamer
valgrind good
created #5754162
gstreamer
valgrind ugly
 
  Integrate
created #5754165
cross-android universal examples
created #5754167
gst-ios-13.2
cross-ios universal examples
created #5754166
documentation