cochran_emc.h: packed structs

[Lubomir I. Ivanov]

} __attribute__((packed));
is unreliable with MINGW > 4.6 up until 4.8.2 - the one i have bundled
with the Qt 5.3.

padding is added regardless and there is high potential for the code to break.

one solution would be to branch / use "#pragma pack()" which is an
MSVC thing but recent MINGW supports it.

i'm CCing John Van Ostrand as the author, and posting this message so
that everyone knows.
perhaps there is a need for a SUBSURFACE_PACK_STRUCT(x) macro of sorts.

lubomir
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