X68000 Game Launcher - #3: Bitmap loader

The header includes these common files:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
 
#include "bmp.h"

The header bmp.h defines bitmap data structures and a function prototypes for the bitmap library:

/*
 bmp.h - C prototypes for the simple Windows BMP file loader
 based on the example at:
 https://elcharolin.wordpress.com/2018/11/28/read-and-write-bmp-files-in-c-c/
 
 Modified for use on PC98Launcher by John Snowdon (2020).
 
*/
 
#define BMP_FILE_SIG_OFFSET		0x0000 // Should always be 'BM'
#define BMP_FILE_SIZE_OFFSET		0x0002 // Size of file, including headers
#define DATA_OFFSET_OFFSET 		0x000A // How many bytes from 0x0000 the data section starts
#define DIB_HEADER_OFFSET		0x000E // Where the DIB header can be found
#define DIB_HEADER_SIZE			4 // DIB header size field is 4 bytes
#define WIDTH_OFFSET 			0x0012 // Where we can find the x-axis pixel size
#define HEIGHT_OFFSET 			0x0016 // Where we can find the y-axis pixel size
#define BITS_PER_PIXEL_OFFSET	0x001C // Where we can find the bits-per-pixel number
#define COMPRESS_OFFSET		0x001E // Where we can find the compression flag
#define COLOUR_NUM_OFFSET		0x002E // Where we can find the numbers of colours used in the image
#define COLOUR_PRI_OFFSET		0x0032 // Where we can find the number of the 'important' colour ???
#define PALETTE_OFFSET			0x0036 // Where the colour palette starts, for <=8bpp images.
#define HEADER_SIZE 				14
#define INFO_HEADER_SIZE 		40
#define BMP_1BPP				1
#define BMP_4BPP				4
#define BMP_8BPP				8	
#define BMP_16BPP				16
#define BMP_UNCOMPRESSED		0
#define BMP_VERBOSE			0 // Enable BMP specific debug/verbose output
#define BMP_OK					0 // BMP loaded and decode okay
#define BMP_ERR_NOFILE			-1 // Cannot find file
#define BMP_ERR_SIZE			-2 // Height/Width outside bounds
#define BMP_ERR_MEM			-3 // Unable to allocate memory
#define BMP_ERR_BPP				-4 // Unsupported colour depth/BPP
#define BMP_ERR_READ			-5 // Error reading or seeking within file
#define BMP_ERR_COMPRESSED	-6 // We dont support comrpessed BMP files
#define BMP_ERR_FONT_WIDTH		-7 // We dont support fonts of this width
#define BMP_ERR_FONT_HEIGHT	-8 // We dont support fonts of this height
#define BMP_FONT_MAX_WIDTH		8
#define BMP_FONT_MAX_HEIGHT	16
#define BMP_FONT_PLANES		4 // Number of colour planes per pixel
 
// ============================
//
// A single palette entry of 8bit r, g and b values
//
// ============================
typedef struct pal_entry {
	unsigned char r;
	unsigned char g;
	unsigned char b;
	unsigned char new_palette_entry;
} pal_entry_t;
 
// ============================
//
// BMP image data structure
//
// ============================
typedef struct bmpdata {
	unsigned int 	width;			// X resolution in pixels
	unsigned int 	height;			// Y resolution in pixels
	char			compressed;		// If the data is compressed or not (usually RLE)
	unsigned int	dib_size;			// size of the DIB header
	unsigned char	is_indexed;		// If the image uses a palette table
	unsigned short	colours_offset;
	unsigned int	colours;			// Count of colours used
	unsigned short	bpp;			// Bits per pixel
	unsigned short	bytespp;			// Bytes per pixel
	unsigned int	offset;			// Offset from header to data section, in bytes
	unsigned int 	row_padded;		// Size of a row without padding
	unsigned int 	row_unpadded;	// SIze of a row, padded to a multiple of 4 bytes
	unsigned int 	size;			// Size of the pixel data, in bytes
	unsigned int	n_pixels;			// Number of pixels
	struct pal_entry	palette[256];		// Palette entries for 8bit indexed images
	unsigned char	*pixels;			// Pointer to raw pixels - in font mode each byte is a single pixel
} bmpdata_t;
 
// ============================
//
// BMP state structure
//
// ============================
typedef struct bmpstate {
	unsigned int	width_bytes;
	unsigned int	rows_remaining;	// Total number of rows left to be read
	unsigned char	*pixels;			// Needs to be malloc'ed to the width of a single row of pixels
} bmpstate_t;
 
// ============================
//
// Font data structure
//
// 96 characters, 
// each character is 1 byte wide, and up to 16 (max) rows high 
// each row is 4 planes
// total of 6144 bytes per 96 character font
//
//=============================
typedef struct fontdata {
	unsigned char			width;			// Width of each character, in pixels
	unsigned char			height;			// Height of each character, in pixels
	unsigned char			ascii_start;		// ASCII number of symbol 0
	unsigned char			n_symbols;		// Total number of symbols
	unsigned char			unknown_symbol;	// Which symbol do we map to unknown/missing symbols?
	unsigned char 			symbol[96][16][16]; // Only up to 16px high, 16 px wide fonts
}  fontdata_t;
 
void	bmp_Destroy(bmpdata_t *bmpdata);
void	bmp_DestroyFont(fontdata_t *fontdata);
int 		bmp_ReadFont(FILE *bmp_image, bmpdata_t *bmpdata, fontdata_t *fontdata, unsigned char header, unsigned char palette, unsigned char data, unsigned char font_width, unsigned char font_height);
int 		bmp_ReadImage(FILE *bmp_image, bmpdata_t *bmpdata, unsigned char header, unsigned char palette, unsigned char data);
int 		bmp_ReadImageHeader(FILE *bmp_image, bmpdata_t *bmpdata);
int 		bmp_ReadImagePalette(FILE *bmp_image, bmpdata_t *bmpdata);
int 		bmp_ReadImageData(FILE *bmp_image, bmpdata_t *bmpdata);