If the calling test hasn't set m_iPicResSize, it is set to the
maximum frame size, which takes much longer to initialize than the
current actual frame size.
This reduces the runtime of EncoderInterfaceTest.SkipFrameCheck
in valgrind from 229 seconds to 8 seconds, and the total runtime
of all the test cases in EncoderInterfaceTest from 405 seconds
to 89 seconds.