Scale by 3/8 only if source is multiple of 8 tall.

Scale by 3/8 only if source is multiple of 8 tall.

BUG=libyuv:635
TEST=try bots
R=harryjin@google.com

Committed: https://chromium.googlesource.com/libyuv/libyuv/+/8279df963e10ff586cb43570a6ded61f0c352ad4

[fbarchard1, 2016-09-16 21:22:29]

fbarchard@google.com changed reviewers:
+ harryjin@google.com

[fbarchard1, 2016-09-16 21:22:29]

The old formula:

if (8 * dst_width == 3 * src_width &&	 1428     if (8 * dst_width == 3 *
src_width &&
 1429         dst_height == ((src_height * 3 + 7) / 8)) {

for selecting 3/8 scaler, used the same formula as the buffer calculation would,
ensuring enough destination rows for the odd height remainder.  But to implement
the odd height, the last destination row would sometimes require clamping.

Making the check require exact height ensure the optimized method is only used
if the scale factor is exactly 3 / 8.

Advantage - avoids overread causing memory exception.

Downside is 360p and 1080p will use 3 / 8 on luma, but general purpose scaler on
chroma, causing a perceptual and performance regression.
But 720p and 4k will be ok.

[harryjin, 2016-09-16 21:31:04]

lgtm

[fbarchard1, 2016-09-16 21:57:50]

Description was changed from

==========
Scale by 3/8 only if source is multiple of 8 tall.

BUG=libyuv:635
TEST=try bots
==========

to

==========
Scale by 3/8 only if source is multiple of 8 tall.

BUG=libyuv:635
TEST=try bots
R=harryjin@google.com

Committed:
https://chromium.googlesource.com/libyuv/libyuv/+/8279df963e10ff586cb43570a6d...
==========

[fbarchard1, 2016-09-16 21:57:51]

Committed patchset #2 (id:20001) manually as
8279df963e10ff586cb43570a6ded61f0c352ad4 (presubmit successful).