This change allows the compiler to keep the loop variable (readPtr) in a
register and therefore avoid cache miss in what is essentially a more general
memcpy.

By analysing the assembly generated by both gcc 6.3.1, gcc 4.8.5 and clang 5.0
I found that these compilers interpret the mutable reference such that it
has to be written back into memory in every iteration.

The performance regression  was when upgrading the compilers for Foundry's
Nuke. Our original build of OpenEXR 2.2.0 built with GCC 4.1.2 did not exhibit
this behaviour. It yielded significant speed up in Nuke's writing speed.

Signed-off-by: Gyula Gubacsi <[email protected]>