The issue was that some `material_index` were `-1`. This meant that
the rendering code was accessing out-of-bounds memory to read
from the material pool.

The fix adds an assert to the `gpencil_material_resources_get` and
clamps the `material_index` to >= 0 to ensure that the material is
valid. It also changes the default read value for the material indices
in `create_curves_outline` and `retrieve_visible_strokes` to be 0
instead of -1.

@@ -791,3 +791,3 @@
     visible_strokes.foreach_index([&](const int stroke_i, const int pos) {
       const IndexRange points = points_by_curve[stroke_i];
-      const int material_index = stroke_materials[stroke_i];
+      /* The material index shouldn't be negative. We clamp it here to avoid crashing in the