Remove the call to 'abs' since unsigned values cannot be negative

[juergenhoetzel]

The C11 standard states that for unsigned integers modulo wrapping is
the defined behavior and the term overflow never applies: "a
computation involving unsigned operands can never overflow."

Fixes warnings (clang):

ibbmp.c
../../home/juergen/shared/low-level-programming/sepia_filter/libbmp/libbmp.c:164:27: warning: taking the absolute value of unsigned type 'unsigned long' has no effect [-Wabsolute-value]
                fwrite (img->img_pixels[abs (offset - y)], sizeof (bmp_pixel), img->img_header.biWidth, img_file);
                                        ^
../../home/juergen/shared/low-level-programming/sepia_filter/libbmp/libbmp.c:164:27: note: remove the call to 'abs' since unsigned values cannot be negative
                fwrite (img->img_pixels[abs (offset - y)], sizeof (bmp_pixel), img->img_header.biWidth, img_file);
                                        ^~~
../../home/juergen/shared/low-level-programming/sepia_filter/libbmp/libbmp.c:210:30: warning: taking the absolute value of unsigned type 'unsigned long' has no effect [-Wabsolute-value]
                if (fread (img->img_pixels[abs (offset - y)], sizeof (bmp_pixel), items, img_file) != items)
                                           ^
../../home/juergen/shared/low-level-programming/sepia_filter/libbmp/libbmp.c:210:30: note: remove the call to 'abs' since unsigned values cannot be negative
                if (fread (img->img_pixels[abs (offset - y)], sizeof (bmp_pixel), items, img_file) != items)
                                           ^~~
2 warnings generated.

[0xBERNDOG]

This isn't functionally correct. If offset = 0 then the value wraps around starting from the max value of size_t

[0xBERNDOG]

Not functionally correct, see #62