Added check to make sure maximum buffer size not exceeded

Added code to clip the buffer level to the maximum buffer
size. Without this the buffer level would increase
unchecked.

This bug was found when encoding an essentially static
scene at 2Mb/s. The encoder is unable to generate frames
consistent with the high data-rate because Q bottoms out
at Qmin.

As frames generated are consistently undersized the buffer
level increases and does not get checked against the
maximum size specified by the user (or default).

Change-Id: Id8a3c6323d3246da50f7cb53ddbf78b5528032c6