lavd: add avdevice_app_to_dev_control_message API

New API allows to send messages from application to devices.

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>
lavd: add avdevice_dev_to_app_control_message API

New API allows to send messages from devices to application.

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>
lavd: add opengl device

It can render to OpenGL context provided by application or into SDL window

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>
Merge remote-tracking branch 'lukaszmluki/master'

* lukaszmluki/master:
  lavd: add opengl device
  lavd: add avdevice_dev_to_app_control_message API
  lavd: add avdevice_app_to_dev_control_message API

Merged-by: Michael Niedermayer <michaelni@gmx.at>