Capture at least 2 frames

Current post processing scripts assume at least 2 frames are captured.
With the rear pinephone camera the automatic gain control will always
raise the gain high enough that the frame count will be 2 or higher. For
the front camera the gain might be zero causing the burst to be 1 frame
long.

src/io_pipeline.c

@@ -388,7 +388,7 @@ capture(MPPipeline *pipeline, const void *data)
         // for manual gain you can go up to 11 frames
         gain = mp_camera_control_get_int32(info->camera, V4L2_CID_GAIN);
         gain_norm = (float)gain / (float)info->gain_max;
-        burst_length = (int)(sqrt(gain_norm) * 10) + 1;
+        burst_length = (int)fmax(sqrt(gain_norm) * 10, 1) + 1;
         captures_remaining = burst_length;
 
         // Change camera mode for capturing