docs/random/bbb/subtitles: Add some first mind rumblings on proper subtitle support.

Original commit message from CVS:
* docs/random/bbb/subtitles:
Add some first mind rumblings on proper subtitle support.

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+Subtitles
+=========
+
+1. Problem
+GStreamer currently does not support subtitles.
+
+2. Proposed solution
+  - Elements
+  - Text-overlay
+  - Autoplugging
+  - Scheduling
+  - Stream selection
+
+The first thing we'll need is subtitle awareness. I'll focus on AVI/MKV/OGM
+here, because I know how that works. The same methods apply to DVD subtitles
+as well. The matroska demuxer (and Ogg) will need subtitle awareness. For
+AVI, this is not needed. Secondly, we'll need subtitle stream parsers (for
+all popular subtitle formats), that can deal both with parsed streams (MKV,
+OGM) as well as .sub file chunks (AVI). Sample code is available in
+gst-sandbox/textoverlay/.
+
+Secondly, we'll need a textoverlay filter that can take text and video and
+blits text on video. We have several such elements (e.g. the cairo-based
+element) in gst-plugins already. Those might need some updates to work
+exactly as expected.
+
+Thirdly, playbin will need to handle all that. We expect subtitle streams
+to end up as subimages or plain text (or xhtml text). Note that playbin
+should also allow access to the unblitted subtitle as text (if available)
+for accessibility purposes.
+
+A problem popping up is that subtitles are no continuous streams. This is
+especially noticeable in the MKV/OGM case, because there the input of data
+depends on the other streams, so we'll only notice delays inside an element
+when we've received the next data chunk. There are two possible solutions:
+using timestamped filler events or using decoupled subtitle overlay elements
+(bins, probably). The first has as a difficulty that it only works well in
+the AVI/.sub case, where we will notice discontinuities before they become
+problematic. The second is more difficult to implement, but works for both
+cases.
+A) fillers
+Imagine that two subtitles come after each other, with 10 seconds of no-data
+in between. By parsing a .sub file, we would notice immediately and we could
+send a filler event (or empty data) with a timestamp and duration in between.
+B) decoupled
+Imagine this text element:
+------------------------------
+video ----- | actual element |out
+|        /  -----------------|
+text - -                     |
+------------------------------
+where the text pad is decoupled, like a queue. When no text data is available,
+the pad will have received no data, and the element will render no subtitles.
+The actual element can be a bin here, containing another subtitle rendering
+element. Disadvantage: it requires threading, and the element itself is (in
+concept) kinda gross. The element can be embedded in playbin to hide this
+fact (i.e. not be available outside the scope of playbin).
+Whichever solution we take, it'll require effort from the implementer.
+Scheduling (process, not implementation) knowledge is assumed.
+
+Stream selection is a problem that audio has, too. We'll need a solution for
+this at the playback bin level, e.g. playbin. By muting all unused streams
+and dynamically unmuting the selected stream, this is easily solved. Note
+that synchronization needs to be checked in this case. The solution is not
+hard, but someone has to do it.
+
+3. Written by
+Ronald S. Bultje <rbultje@ronald.bitfreak.net>, Dec. 25th, 2004