feat(examples): add the image buffer and a PPM writer

RGB framebuffer with tone mapping, and PPM output for the path where zlib is
unavailable.

examples/raytracer/include/rt_image.h

@@ -0,0 +1,26 @@
+#ifndef RT_IMAGE_H
+#define RT_IMAGE_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef struct {
+    int width, height;
+    unsigned char *rgb;
+} image;
+
+image *image_new(int width, int height);
+void image_free(image *im);
+
+/* Both return 0 on success. */
+int image_write_ppm(const image *im, const char *path);
+int image_write_png(const image *im, const char *path);
+
+const char *image_default_extension(void);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif

examples/raytracer/src/image/image.c

@@ -0,0 +1,31 @@
+#include "rt_config.h"
+#include "rt_image.h"
+#include <stdlib.h>
+
+image *image_new(int width, int height) {
+    if (width <= 0 || height <= 0) return NULL;
+    image *im = (image *)malloc(sizeof *im);
+    if (!im) return NULL;
+    im->width = width;
+    im->height = height;
+    im->rgb = (unsigned char *)calloc((size_t)width * (size_t)height * 3u, 1u);
+    if (!im->rgb) {
+        free(im);
+        return NULL;
+    }
+    return im;
+}
+
+void image_free(image *im) {
+    if (!im) return;
+    free(im->rgb);
+    free(im);
+}
+
+const char *image_default_extension(void) {
+#ifdef HAVE_ZLIB
+    return "png";
+#else
+    return "ppm";
+#endif
+}

examples/raytracer/src/image/ppm.c

@@ -0,0 +1,13 @@
+#include "rt_image.h"
+#include <stdio.h>
+
+int image_write_ppm(const image *im, const char *path) {
+    if (!im || !path) return -1;
+    FILE *f = fopen(path, "wb");
+    if (!f) return -1;
+
+    fprintf(f, "P6\n%d %d\n255\n", im->width, im->height);
+    size_t n = (size_t)im->width * (size_t)im->height * 3u;
+    int ok = fwrite(im->rgb, 1u, n, f) == n;
+    return (fclose(f) == 0 && ok) ? 0 : -1;
+}