VP8: Allocate cpi->mt_current_mb_col array lazily

VP8: Allocate cpi->mt_current_mb_col array lazily

Add the mt_current_mb_col_size field to VP8_COMP to record the size of
the mt_current_mb_col array.

Move the allocation of the mt_current_mb_col array from
vp8_alloc_compressor_data() to vp8_encode_frame(), where the use of
mt_current_mb_col starts. Allocate mt_current_mb_col right before use
if mt_current_mb_col hasn't been allocated or if the current size is
incorrect.

Move the deallocation of the mt_current_mb_col array from
dealloc_compressor_data() to vp8cx_remove_encoder_threads().

Move the TODO(https://crbug.com/1486441) comment from
vp8/encoder/onyx_if.c to vp8/vp8_cx_iface.c.

Change-Id: Ic5a0793278c2cc94876669aaa0dd732412876673