Build issue with clang6

[adriaandegroot]

struct scanLineHeader in aces_formatter.cpp uses int32 for its fields, but is initialized with uint32 values. This causes build failures with strict C++ compilers (Clang6 defaults to C++17, I think). Error message is:

aces_formatter.cpp:255:24: error: non-constant-expression cannot be narrowed from type 'unsigned int' to 'int32' (aka 'int') in initializer list [-Wc++11-narrowing]
        scanLineHeader sh = { yMin + row, scanLineSize };

This patch silences the build error, but I can't really tell if it's correct -- it would seem to me that coordinate and size are both unsigned by nature.

+--- aces_formatter.cpp.orig    2018-06-23 19:34:29 UTC
++++ aces_formatter.cpp
+@@ -107,8 +107,8 @@
+ #include <cassert>
+ 
+ struct scanLineHeader {
+-      int32   yCoordinate;
+-      int32   pixelDataSize;
++      uint32  yCoordinate;
++      uint32  pixelDataSize;
+ };
+ 
+