Added drop_frame support in multi-resolution encoder.

If one frame is dropped at a lower-resolution level, the next
upper-resolution level encoder needs to encode that frame
independently without any lower-resolution level motion
information.

Another issue is that if one frame is dropped at some but not all
resolution levels, a frame after that one may use different set
of reference frames at different resolution levels. This reference
frame asynchronization could degrade motion search precision in
upper-resolution level encoding, which uses lower-resolution level
motion result. This change compares the lower-resolution and upper-
resolution level's reference frames. If they are not the same, the
upper-resolution level encoder can not use lower-resolution level
motion result.

Change-Id: I61afa4f313630e75b7cbdd5742e230e8724a988a