If we use the x264enc encoder to encode a stream, then videoconvert
would convert the BGRx data into Y444, which is the preferred format
for x264enc. However, some decoders particularly the ones that are
h/w based cannot work with Y444 if it was the format used by the
encoder. Therefore, to address these situations, we need a way to
override the format used during the encoding stage which can be
accomplished by using the environment variable introduced in this
patch: SPICE_CONVERTER_PREFERRED_FORMAT.

For example, using NV12 as the output format for the videoconvert
element would allow us to pair a s/w based encoder (such as x264enc)
with a h/w based decoder (such as msdkh264dec) for decoding the
stream as most h/w based decoders only work with NV12 format given
its popularity.

Note that choosing an encoder format such as NV12 over Y444 would
probably result in decreased video quality although it would be
compatible with more decoders. Ideally, the client and server need
to negotiate a suitable format dynamically but the current
capabilities do not allow for such exchange.

Cc: Frediano Ziglio <freddy77@gmail.com>
Cc: Dongwon Kim <dongwon.kim@intel.com>
Based-on-patch-by: Hazwan Arif Mazlan <hazwan.arif.mazlan@intel.com>
Signed-off-by: Jin Chung Teng <jin.chung.teng@intel.com>
Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Acked-by: Frediano Ziglio <freddy77@gmail.com>