Write the scene in writer_png

- Add pixels to Scene struct

src/scene.h

@@ -17,6 +17,9 @@ typedef struct _Scene
 {
     int height, width;      /* Pixel dimensions. */
     Camera *camera;
+
+    int is_rendered;
+    Color *pixels;
 } Scene;
 
 

src/writer_png.c

@@ -7,6 +7,7 @@
 
 
 #include <setjmp.h>
+#include <stdlib.h>
 
 #include <png.h>
 
@@ -25,30 +26,60 @@ static void png_user_warning(png_structp png, png_const_charp msg);
 int
 write_scene_png(Scene *scene, FILE *file)
 {
-    if (!file) {
+    if (!scene->is_rendered) {
         return -1;
     }
 
+    if (!file) {
+        return -2;
+    }
+
     // Set up the PNG data structures. libpng requires two: a PNG object and an info object.
     png_structp png = png_create_write_struct(PNG_LIBPNG_VER_STRING, NULL, png_user_error, png_user_warning);
     if (!png) {
-        return -2;
+        return -3;
     }
 
     png_infop png_info = png_create_info_struct(png);
     if (!png_info) {
-        return -3;
+        return -4;
     }
 
     // Set up libpng error handling. If an error occurs, libpng will longjmp back here... (Wat.)
     if (setjmp(png_jmpbuf(png))) {
         png_destroy_write_struct(&png, &png_info);
-        return -4;
+        return -5;
     }
 
     // Set up output.
     png_init_io(png, file);
 
+    // Write it!
+    png_byte **rows = malloc(png_sizeof(png_bytep) * scene->height);
+    for (int y = 0; y < scene->height; y++) {
+        rows[y] = malloc(png_sizeof(png_byte) * scene->width * 4);
+        if (rows[y] == NULL) {
+            // TODO: DANGER! WILL ROBINSON!
+        }
+        for (int x = 0; x < scene->width; x += 4) {
+            rows[y][x] = scene->pixels[y * scene->height + x].red;
+            rows[y][x+1] = scene->pixels[y * scene->height + x].green;
+            rows[y][x+2] = scene->pixels[y * scene->height + x].blue;
+            rows[y][x+3] = scene->pixels[y * scene->height + x].alpha;
+        }
+    }
+
+    png_write_image(png, rows);
+
+    for (int y = 0; y < scene->height; y++) {
+        free(rows[y]);
+    }
+    free(rows);
+
+    // Clean up!
+    png_write_end(png, png_info);
+    png_destroy_write_struct(&png, &png_info);
+
     // TODO: Return number of bytes written.
     return 0;
 }