Sunxi NV12 detiling

[Giulio Benetti]

Hello Nicolas, All,

I'm dealing with Cedrus on Linux 5.15.7 with Gstreamer 1.19.3.1-git(the 
latest main branch) and I've got to the point that on A13 and A20 I have 
to detile the output of Sunxi video-engine, since now it's done in 
software and this causes:
```
videodecoder 
gstvideodecoder.c:3670:gst_video_decoder_clip_and_push_buf:<v4l2slh264dec0> 
Dropping frame due to QoS.
```
using kmssink. And I get an entire frame every 5 seconds.

I've found this [1] IRC discussion where you state that kmssink support 
is missing for GST_VIDEO_FORMAT_NV12_32L32. Basically what I understand 
is that we need a special treatment for A13 and A20, since from >= A33 
we have the support for the untiled output(still not tried, but I have 
to do it with A64).

Where can I begin from to implement the kmssink support for detiling? 
There is something I can imitate?
Can you or someone else point me more or less where to start working?

[1]: 
https://oftc.irclog.whitequark.org/linux-sunxi/2021-07-13#1626188848-1626188913;

Thanks in advance
Best regards
-- 
Giulio Benetti
Benetti Engineering sas