Text overlays don't work on i.MX6 in certain pipelines

[Toon Heyrman]

I discovered something extra:
If I use a imxipuvideotransform in front of the videofilter and force the
transform to take a copy of the buffer by setting the output caps a little
bit different (height from 768px to 752px), it works always. 
So there is definitely something going wrong with the buffers.

> That's sounds like a missing cache flush on map. This is on the
> gstreamer-imx side though, as this should be handled by the GstMemory
> implementation when the buffer is mapped.

To flush the map shall we use msync? 
And is it right after the mmap? 
https://github.com/Freescale/gstreamer-imx/blob/master/src/v4l2video/v4l2_buffer_pool.c#L174
<https://github.com/Freescale/gstreamer-imx/blob/master/src/v4l2video/v4l2_buffer_pool.c#L174>  




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