RD costs were local to MACROBLOCK data and had to be copied all the
time to each thread's MACROBLOCK data. Tables moved to a common place
and only pointers are setup for each encoding thread.

vp8_cost_tokens() generates 'int' costs so changed all types to be
int (i.e. removed unsigned).

NOTE: Could do some more cleaning in vp8cx_init_mbrthread_data().

Change-Id: Ifa4de4c6286dffaca7ed3082041fe5af1345ddc0