Fix have_topright check in check_intra_mode().

Prevents reading from uninitialized pointer 'top'.
Don't backup intra pre-lf intra pred borders for last SB row.

The frame height might not be a multiple of 64, and so this data might
be outside the allocated buffers of the image. Since this is the last
row of the image, we therefore skip backing up of the pre-lf intra pred
data.
Initial inter frame bitstream parsing.

First block (of akiyo @ 100kbps) of first inter frame parses correctly.
Actual reconstruction not yet implemented. Sub8x8 inter mode coding is
also not yet implemented.
diag_downleft: re-roll 8x8 16x16 and add 32x32.
diag_downright: re-roll 8x8 16x16 and add 32x32.
(untested): make use of mem{cpy,set} in diag_down{left,right}.
vert_right: re-roll 8x8 16x16 and add 32x32.
tmp: disable vla