Update tilemap tests for renderer backends

Migrate tilemap tests to pointer-based game globals and current loader signatures, initialize the renderer callback used by image comparisons, and refresh the character fixture schema. Preserve loaded tileset names by removing a duplicate heap allocation.

Co-authored-by: Codex (GPT-5) <noreply@openai.com>
Co-Authored-By: Andrew Kesterson <andrew@aklabs.net>

tests/assets/testcharacter.json

@@ -1,6 +1,9 @@
 {
     "name": "testcharacter",
     "movementspeed": 1,
+    "speedtime": 1,
+    "speed_x": 0.20,
+    "speed_y": 0.20,
     "acceleration_x": 0.20,
     "acceleration_y": 0.20,
     "sprite_mappings": [