Fix bug in calculating number of mbs with scaling.

Correct calculation of number of mbs in two pass code when
frame resizing is enabled. Always use initial number of mbs if
scaling is enabled, as this is what was used in the first pass.

Change-Id: I49a4280ab5a8b1000efcc157a449a081cbb6d410
vp9_ethread: move filter_cache out of RD_OPT struct

Similar to mask_filter, the filter_cache in RD_OPT struct can be
moved out, and declared as a local variable since it is only
used in pick_inter_mode functions.

Change-Id: I412b99cca82bade07ac912064ec03dd1de6b2c17
Merge "Fix bug in calculating number of mbs with scaling."
Merge "vp9_ethread: move filter_cache out of RD_OPT struct"