Palette: modify the context slightly

Use the number of pixels in a block as context, rather than the bsize
itself. The rectangular blocks therefore share the same context, e.g.
BLOCK_8X16 and BLOCK_16X8.

The number of contexts is reduced from 10 to 7.
Almost no coding performance changes.

Change-Id: Ib3241194580c2b93ad0e953957cdc9188393d055