ENH: explicit cast to avoid narrow conversion in internal libsiftfast

Following recent commits on the ITK side which use clang-modernize to insert
missing override keyword, cf:

https://github.com/InsightSoftwareConsortium/ITK/commit/10757725be9cbe68ab2903c9cd4b76b738070e63

I had to compile OTB trunk using clang and  c++11 flags:

mkdir OTB-newclang
cd OTB-newclang/
CC=/usr/bin/clang CXX=/usr/bin/clang++ CXXFLAGS="-std=c++11 -stdlib=libstdc++" ccmake -DCMAKE_EXPORT_COMPILE_COMMANDS:BOOL=ON ../../../src/OTB

Internal libsiftfast does not compile as clang report error "error: argument evaluates to XXX
which cannot be narrowed to type YYY"

I added explicit cast to int in that cases.