Reduce memory allocation in alloc_mode_context()
Reduce memory allocation in alloc_mode_context()

Before this patch we were passing in the number of pixels as
number of 4x4 blocks, thus allocating much more memory than
necessary. With this patch, the heap usage on a CIF test clip
drops from 130MB to 93MB.

Change-Id: I6078c5ff02a9cf08aa0eb9acd2527ac3b2baf0f7

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom