1. Export fps related API so that subsystem implementation no longer need to know about details in encoder structure.
2. Discard frameList dictionary.
The 'value' in this dictionary is never used and not properly free'ed when client is disconnected.
The dictionary was used to calculate 'inflight' frame count. Once an ACK is received from client, an item in the dictionary is removed.
We then calculate 'inflight' frame by the count of the items in the dictionary.
However, some rdp clients (win7 mstsc) skips frame ACK if it is inactive, ACK of some frame would actually never arrive.
We actually don't need the dictionary. We only need to record the latest acknowledged frame id, and the difference between last sent frame id is the inflight frame count.
3. Minor fix in default fps calculation. encoder->frameAck is wrongly used as integer while it's actually bool flag.
