v4l2-renderer: receive a mapped address and a stride of output buffer.

In order to support read_pixels, we need to be able to access the output
buffer from user space. We should pass not only DMABUF, but also its
stride and mapped address.
v4l2-renderer: pass v4l2_bo_state to set_output_buffer().

We should pass not only DMABUF, but also stride of the output buffer.
v4l2-renderer: support read_pixels.

For now, we do memcpy().