add dummy image output

src/main.c

@@ -1,6 +1,29 @@
+#include <err.h>
+#include <stdio.h>
+#include <stdlib.h>
+
 #include "image.h"
 #include "ppm.h"
 
-int main(void) {
-    return 0;
+static void fill_image(struct image *image) {
+    for (size_t i = 0; i < image->h; ++i) {
+        for (size_t j = 0; j < image->w; ++j) {
+            struct pixel *p = &image->buf[i * image->w + j];
+            p->r = 255 * i / image->h;
+            p->g = 255 * j / image->w;
+            p->g = 255 * (i + j) / (image->h + image->w);
+        }
+    }
+}
+
+int main(void) {
+    struct image *image = create_image(960, 540);
+    if (!image)
+        err(EXIT_FAILURE, "could not allocate image");
+
+    fill_image(image);
+    print_ppm(image, stdout);
+    destroy_image(image);
+
+    return EXIT_SUCCESS;
 }