[Bug 749243] textoverlay: gltestsrc ! textoverlay ! fakesink does not work

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=749243

--- Comment #2 from Nicolas Dufresne (stormer) <nicolas.dufresne at collabora.co.uk> ---
I must admit I didn't notice the huge hack later to try deal with fakesink in a
special way. I asked around to understand this hack, and here's my conclusion.

When caps feature where introduced, caps ANY has been defined to also mean
CapsFeature ANY. This was in fact an ABI break as a pipeline like:

  textoverlay ! fakesink
  textoverlay ! appsink

Would attach the composition instead of doing a blit (there is a chance that an
old application would loose the text). It seems it was decided to fix the ABI
break semantically, which resulted in a big hack to try and guess if downstream
is appsink or fakesink. My patch breaks this hack and will upload again. I'll
mark the patch as need-work for this reason.

Currently that hack is wrong, as it may fail the negotiation instead of
uploading. With the split GL elements we can now have:

  gltestsrc ! textoverlay

In which case the textoverlay cannot downgrade to bliting, since in bliting
mode it does not support memory:* (in this case it's GLMemory). Failing is
wrong, because the downstream caps explictly says that it does not matter what
meta are being sent.

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