[Bug 766381] v4l2object: use G_SELECTION instead of G_CROP in gst_v4l2_object_acquire_format

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=766381

--- Comment #2 from Philipp Zabel <p.zabel at pengutronix.de> ---
Cropping pertains to the input image. Since mem2mem devices get their input
images fed into the OUTPUT v4l2 queue, cropping the input frames has to be set
there. In the decoder case we don't want to crop the input, though. We just
happen to produce output frames that are larger than the input image dimensions
by some padding. That is described by the compose rectangle of the CAPTURE v4l2
queue. See the right side of figure 1.2 in the selection API chapter [1].
The compose rectangle that describes the "whole picture" according to the spec
is the V4L2_SEL_TGT_COMPOSE_DEFAULT [2].

[1] https://linuxtv.org/downloads/v4l-dvb-apis/selection-api.html
[2] https://linuxtv.org/downloads/v4l-dvb-apis/apb.html#v4l2-selection-targets

Cropping in the v4l2 sense would be if you ordered the decoder to only decode
part of the image.

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