Added H264ID and H265ID consts and added logic to select this const for use in encoder based on mediaType param in NewEncoder. Also now
declaring PMT in NewEncoder so that we can set streamID correctly based on mediaType.
Completed test TestFindPid to validate function of FindPid func in mpegts.go. Through this
process, it was found that there was a bug in this func, whereby the returned packet was
not complete due to indexing issues. The bug is fixed and the test passes.