Improves MJPEG video encoding

As discussed in https://github.com/raspberrypi/userland/issues/208, the MJPEG encoder may deadlock
if the FIFO is full on shutdown. This change increases the encoder buffer size,
which helps to keep the FIFO size down.

With this change, the following program exits cleanly. Previously, it would
lock up the camera, requiring a reboot.

import picamera
import time

camera = picamera.PiCamera()
camera.resolution = (1296, 972)
camera.framerate = 15

camera.start_recording('test.mjpeg', format='mjpeg', quality=0)
time.sleep(5)
camera.stop_recording()