+        guint8 r;
+        guint8 g;
+        guint8 b;
+        guint8 a;

I would recommend against this approach, instead, use
guint8 rgba[4];

and replace
+            _vertices[i].r = pixels[0];
+            _vertices[i].g = pixels[1];
+            _vertices[i].b = pixels[2];
+            _vertices[i].a = pixels[3];

with
for (int p = 0; p < 4; p++) {
  _vertices[i].rgba[p] = pixels[p];
}

It generally ends up being less work and less error prone.