Safely check if adding two integers will overflow (no undefined behavior, compiler removing the code, etc...) and output result. */

Must have enough bits available with ensureBits */

in an idat chunk, each scanline is a multiple of 8 bits, unlike the lodepng output buffer, and in addition has one extra byte per line: the filter byt

calculate bits per pixel out of colortype and bitdepth*/

NOTE: when realloc returns NULL, it leaves the original memory untouched */