gl: document GST_GL_* environment variables

https://bugzilla.gnome.org/show_bug.cgi?id=733245
mpegtspacketizer: avoid timestamp overflows

Cause timing to break in the pipeline that can lead to a stall

https://bugzilla.gnome.org/show_bug.cgi?id=733837
dashdemux: return the error to the source immediately to stop faster

When flushing, this will prevent dashdemux from trying to download more
fragments or more chunks of the same fragment before stopping.

Also improves the error handling to not transform everything non-ok into
an error.

https://bugzilla.gnome.org/show_bug.cgi?id=734014
glupload: don't determine if frame needs upload by pointer compare

https://bugzilla.gnome.org/show_bug.cgi?id=734269
gl/docs: remove superflous 'the'
dashdemux: support downloading segments in reverse order

When a seek with a negative rate is requested, find the target
segment where gstsegment.stop belongs in and then download from
this segment backwards until the first segment.

This allows proper reverse playback.