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