Fix write primitive type enum

Adds a translation from cgltf_primitive_type to gltf primitive rendering
mode in the writer.

cgltf_write.h

@@ -358,6 +358,21 @@ static int cgltf_int_from_component_type(cgltf_component_type ctype)
  }
 }
 
+static int cgltf_int_from_primitive_type(cgltf_primitive_type ctype)
+{
+ switch (ctype)
+ {
+ case cgltf_primitive_type_points: return 0;
+ case cgltf_primitive_type_lines: return 1;
+ case cgltf_primitive_type_line_loop: return 2;
+ case cgltf_primitive_type_line_strip: return 3;
+ case cgltf_primitive_type_triangles: return 4;
+ case cgltf_primitive_type_triangle_strip: return 5;
+ case cgltf_primitive_type_triangle_fan: return 6;
+ default: return -1;
+ }
+}
+
 static const char* cgltf_str_from_alpha_mode(cgltf_alpha_mode alpha_mode)
 {
  switch (alpha_mode)
@@ -453,7 +468,7 @@ static void cgltf_write_asset(cgltf_write_context* context, const cgltf_asset* a
 
 static void cgltf_write_primitive(cgltf_write_context* context, const cgltf_primitive* prim)
 {
- cgltf_write_intprop(context, "mode", (int) prim->type, 4);
+ cgltf_write_intprop(context, "mode", cgltf_int_from_primitive_type(prim->type), 4);
  CGLTF_WRITE_IDXPROP("indices", prim->indices, context->data->accessors);
  CGLTF_WRITE_IDXPROP("material", prim->material, context->data->materials);
  cgltf_write_line(context, "\"attributes\": {");