Read in pixel array by following these instructions:
1. First, allocate space for m `struct pixel *` values, where m is the height of the image.  Each pointer will eventually point to one row of pixel data.
2. For each pointer you just allocated, initialize it to point to heap-allocated space for an entire row of pixel data.
3. Use the given file and pixel_array_offset to initialize the actual struct pixel values. Assume that `sizeof(struct pixel) == 3`, which is consistent with the bitmap file format.
4. Return the address of the first `struct pixel *` you initialized.