mainly: a few type changes, display: add --2x2D option, operation: add maximum operator.
modified:   crop/CImg.crop.cpp: comment 4 quadrant part and change image type from char to int.
modified:   crop/_info.crop.txt: add optimized compilation (g++ -O3) and other external image format library.
modified:   display/CImg.display.cpp: add 2 images display (--2x2D option and corresponding -i1 and -i2 input file name options).
modified:   display/_info.CImg.display.txt: add optimized compilation (g++ -O3) and other external image format libraries.
modified:   fourier/CImg.Fourier_transform2d.cpp: as a comment, 16bit image conversion to 08bit prior to FFT.
modified:   operation/CImg.operation.cpp: add maximum operation (operator: max or subtraction).
modified:   reduce/CImg.reduce.cpp: change image type from int to float.
Merge branch 'master' of github.com:coupdair/CImg.Program into dev_merge
Conflicts:
	crop/CImg.crop.cpp