[Bug 753824] assrender: render overlay using the video output window size

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=753824

--- Comment #10 from Arnaud Vrac <rawoul at gmail.com> ---
As discussed on IRC, the negotiation issue is a bug in the gl elements which
also affects textoverlay (Bug #753850).

I disabled hinting in libass and I get better results at small sizes.

However, I don't believe it's possible to get pixel perfect rendering with the
current overlay API, because of rounding errors. This is because the overlay
rectangle render geometry is converted to video coordinates. Then the sink will
convert the rectangle render geometry back to window coordinates, resulting in
a loss of precision.

This wouldn't matter if the overlay rectangle had a constant size, however
assrender tries to minimize the composition rectangle size, by computing the
bounding box of the rendered image in the video. Since the bounding box can
change between each frame, this can lead in slight shifts between frames.

Two solutions:
 - add an api to set floating point render size on the overlay rectangles
 - just allocate overlay rectangles of the full frame size, but it wastes a lot
of memory and slows down rendering.

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