Allow connection flag override

[techyian]

A static method is currently being used internally to create connections, and the flags being used are the same as Raspistill/vid. A user should have the flexibility to override the connection flags being used.

I came across a new flag which was added in August 2018 called MMAL_CONNECTION_FLAG_KEEP_PORT_FORMATS. This flag is used to prevent port formats from being copied from an input to output port on creation of the connection.

raspberrypi/userland@25155c0#diff-5e8ea8de7c653ef960b89cfe5ec2e373