OpenEXRCore Deep pixel unpacking optimisation (#2049)

* Refactor UNPACK_SAMPLES macro to allow code reuse.

Breaking each case of the UNPACK_SAMPLES macro into seperate smaller macros
that can be reused to avoid code duplication.

Signed-off-by: Nikolaos Koutsikos <nikolaos.koutsikos@foundry.com>

* Refactor some common code out in a macro

Signed-off-by: Nikolaos Koutsikos <nikolaos.koutsikos@foundry.com>

* Optimise EXRCore Deep pixel unpacking

The functions for doing Deep pixel unpacking, had two branch checks (switch
statements) for checking the pixel and requested data type within the inner
most loop that goes over the pixels of each line. This is definitely
unnecessary, since the data types remain the same. However the compiler
is not able to optimise this, and the generated assembly for the pixel
for loop is massive since it contains all these branch checks.
This commit fixes this issue by moving the two switch statements outside
of the pixel for loop, which makes the compiler able to generate much more
efficient assembly for the pixel unpacking operations.

Signed-off-by: Nikolaos Koutsikos <nikolaos.koutsikos@foundry.com>

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Signed-off-by: Nikolaos Koutsikos <nikolaos.koutsikos@foundry.com>
Co-authored-by: Cary Phillips <cary@ilm.com>

Author: nikos-foundry