Use fewer multithreaded decoders to save memory.
Use fewer multithreaded decoders to save memory.

We run out of memory in 32-bit x86 build, because the AV1Decoder struct
is 16 MB. Although we should definitely make AV1Decoder smaller, we can
save memory by using fewer multithreaded decoders.

This CL creates 3 multithreaded decoders with 2, 4, and 8 threads each.

BUG=aomedia:1870

Change-Id: I3b9e430050a58013f60f54dc021c0c78b1e79c78

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