Initialize graphics context in png_export (guillaumechereau#361)

* Initialize graphics context in png_export

* Signed declaration

doc/cla/individual/madd-games.md

@@ -0,0 +1,11 @@
+UK, 2024-03-18
+
+I hereby agree to the terms of the Goxel Individual Contributor License
+Agreement v1.0.
+
+I declare that I am authorized and able to make this agreement and sign this
+declaration.
+
+Signed,
+
+Mariusz Pilipczuk [email protected] https://github.com/madd-games

src/formats/png.c

@@ -22,6 +22,10 @@
 // XXX: this function has to be rewritten.
 static int png_export(const image_t *img, const char *path, int w, int h)
 {
+    if (!goxel.graphics_initialized) {
+        goxel_create_graphics();
+    }
+
     uint8_t *buf;
     int bpp = img->export_transparent_background ? 4 : 3;
     if (!path) return -1;