fix redundant func in image pre-process

[lexuszhi1990]

Hi, I find a little bit to speed up the code fix the redundant func in image pre-process to make it more efficient... the test result below:

ori:
$ make && ./demo ../model/test.jpg
Scanning dependencies of target demo
[ 33%] Building CXX object CMakeFiles/demo.dir/cpp/DBface.cpp.o
[ 66%] Linking CXX executable demo
[100%] Built target demo
in 0 0.028716
in 0 0.068198
in 0 0.174478

current:
$ make && ./demo ../model/test.jpg
Scanning dependencies of target demo
[ 33%] Building CXX object CMakeFiles/demo.dir/cpp/DBface.cpp.o
[ 66%] Building CXX object CMakeFiles/demo.dir/demo.cpp.o
[100%] Linking CXX executable demo
[100%] Built target demo
in 0 0.028716
in 0 0.068198
in 0 0.174478

which has same results, but it didn't to enumerate all input image by

for (int i = 0; i < c; ++i) {
        for (int j = 0; j < h; ++j) {
            for (int k = 0; k < w; ++k) {