[Fix] (MG_Impl/GLImpl, MG_State/GLState, MG_Test/Texture, scripts/data): keep GENERATE_MIPMAP on the texture, not keyed by GL name

- A process-wide map keyed by GL name gave a deleted texture's flag to the
  next texture created under the recycled name.
- The flag is now texture object state, so it dies with its texture.

scripts/data/structure_baseline.json

@@ -1,5 +1,5 @@
 {
-  "file_excess_lines": 21859,
+  "file_excess_lines": 21854,
   "function_excess_lines": 15516,
   "layering_violations": 63
 }