Fix crash if we open an empy image

On top of an empty image.

This is the second version of this fix, in the first commit I made a
mistake and hashed the matrix (mat) instead of the material (material).

The issue is that we need to make sure that the hash changes when we
open a new image, even if the image is exactly the same, because the
material pointer of the cached render_layers still point to the old
image.

I think a proper solution would be to avoid using pointer for
layer->material but instead an index.  The same could be done for
image->layer, image->camera, etc.

src/layer.c

@@ -57,8 +57,11 @@ uint32_t layer_get_key(const layer_t *layer)
     key = XXH32(&layer->shape, sizeof(layer->shape), key);
     key = XXH32(&layer->color, sizeof(layer->color), key);
     key = XXH32(&mat_key, sizeof(mat_key), key);
-    key = XXH32(&layer->mat, sizeof(layer->mat), key);
     key = XXH32(&layer->mode, sizeof(layer->mode), key);
+
+    // Also hash the material pointer, to avoid some possible crashes with
+    // goxel.render_layers cache.
+    key = XXH32(&layer->material, sizeof(layer->material), key);
     return key;
 }