glupload: provide the output buffer that is rendered into

glupload: provide the output buffer that is rendered into

Allows callers to properly reference count the buffers used for
rendering.

Fixes a redraw race in glimagesink where the previous buffer
(the one used for redraw operations) is freed as soon as the next
buffer is uploaded.

1. glimagesink uploads in _prepare() to texture n
1.1 glupload holds buffer n
2. glimagesink _render()s texture n
3. glimagesink uploads texture n+1
3.1 glupload free previous buffer which deletes texture n
3.2 glupload holds buffer n+1
4. glwindow resize/expose
5. glimagesink redraws with texture n

The race is that the buffer n (the one used for redrawing) is freed as soon as
the buffer n+1 arrives.  There could be any amount of time and number of
redraws between this event and when buffer n+1 is actually rendered and thus
replaces buffer n as the redraw source.

https://bugzilla.gnome.org/show_bug.cgi?id=736740