gstqueuearray: make find() return a 0-based index
gstqueuearray: make find() return a 0-based index

And make the drop() functions expect a 0-based index too,
this addresses a longstanding FIXME. This will not break
backward compatibility, because the drop() functions
were previously only meant to be used with the index
returned by find().

https://bugzilla.gnome.org/show_bug.cgi?id=795156

StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer