BUG: Increased OtsuThreshold computation precision

While debugging differences in the Debian 32-bits platform with SSE disable it
was found that the intermediate computations of the variance difference were
getting truncated in some platforms from 80bits floats used in internal FPU
registers, to 64bits ones used in memory. This truncation resulted in
inconsistent results across platforms.

In order to achieve cross-platform consistency, this patch introduces the use
of the "volatile" modifier in the declaration of intermediate computations of
inter-class variance differences. The "volatile" modifier instructs the
compiler not to keep values in the FPU registers, and instead store them in
memory, where they are actually fitted to the IEEE float/double representation.
This change ensures consistency of computation across platforms.

Change-Id: I2aa8833c06bba1bb9a06ef2a00ef4b2512bd6f46
Merge topic 'debian32'

b3ca416b BUG: Increased OtsuThreshold computation precision