Structure of a BMP Header, and function to write it into a file. 
* 0x4D42 is the magic number (means "BM")
* This my best methods to do it with only 1 write syscall
* WIN_X and WIN_Y corresponds respectively  to the width and the height of the bitmap
* # pragma pack(push, 1) are required to fit the structure as its real size and be well formated
* The 4 in header.bitmap_size corresponds to BPP (bits per pixel, RGBA here)