audiobasesink: Re-work GAP buffer and trick-mode handling

audiobasesink: Re-work GAP buffer and trick-mode handling

In trickmode no-audio mode, or when receiving a GAP buffer,
discard the contents and render as a GAP event instead.

Make sure when rendering a gap event that the ring buffer will
restart on PAUSED->PLAYING by setting the eos_rendering flag.

This mostly reverts commit 8557ee and replaces it. The problem
with the previous approach is that it hangs in wait_preroll()
on a PLAYING-PAUSED transition because it doesn't commit state
properly.

https://bugzilla.gnome.org/show_bug.cgi?id=735666