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enums for BLOCK_SIZE, TX_SIZE and PREDICTION_MODE.

Note: These were converted to #defines earlier to save on memory:
https://chromium-review.googlesource.com/#/c/269854/

But we, instead, use attribute 'packed' (see here:
https://gcc.gnu.org/onlinedocs/gcc/Common-Type-Attributes.html#Common-Type-Attributes)
to ensure that these enums use the smallest possible integer type,
and so use smallest memory when used in structs/arrays etc.

Change-Id: If1fc136686b28847109c9f3a06f8728165e7e475