Fix the negative image only converting 1/3 of the image

[Absolute-Arthur]

This only modifies the negative_image.c

Currently, the width and height values are taken straight from the header, and used directly, assuming that 1 byte = 1 pixel. This results in corrupted image results, which, when viewed with a viewer handling this corruption, reveals that only 1/3 of the original image is treated and put in the result image.
The reason is that each pixel has 3 bytes, corresponding to the 3 colors of RGB.
Simply multiplying these values by 3 fixes this, allowing the memory allocations to ask for 3 times the size and treating the whole image.

[abhijitnathwani]

Hi @Absolute-Arthur
Thank you for the PR. Can you please share the image where you faced this issue ?
Thanks,
Abhijit

[Absolute-Arthur]

It is really any BMP image I had, be it an image of a banana or an image of a landscape (converted using Paint).
Unfortunately Github does not support BMP attachements so I can't show you sorry.
I've also just realized that I did a small mistake in my fix, and multiplied the sizes by 9 instead of 3 (3*3=9), resulting in images being 3 times bigger and filled with nothing.