Downscaled images broken in Firefox built with OPTIMIZED_CFLAGS

[Bengt Ahlgren]

I have started to see this problem (now with FF 33.0) with white stripes
in downscaled images:

https://bugzilla.mozilla.org/show_bug.cgi?id=810604

That bug report says that the problem appears when compiling with -O3 -
-O2 (or -Os) should be ok.  My FF indeed seems to be built with -O3 when
using standard options.  I rebuilt with unchecked option
OPTIMIZED_CFLAGS, and the problem disappeared.

A workaround is to turn off image.high_quality_downscaling.enabled.

(I run 9.3-REL amd64, but build packages on 9.1-REL using poudriere.)

Bengt