Fix Grid to support None/null texture and fix error message bug

- Allow Grid to be created with None as texture parameter
- Use default cell dimensions (16x16) when no texture provided
- Skip sprite rendering when texture is null, but still render colors
- Fix issue #77: Corrected copy/paste error in Grid.at() error messages
- Grid now functional for color-only rendering and entity positioning

Test created to verify Grid works without texture, showing colored cells.

Closes #77

src/UIGrid.h

@@ -21,6 +21,9 @@ class UIGrid: public UIDrawable
 {
 private:
     std::shared_ptr<PyTexture> ptex;
+    // Default cell dimensions when no texture is provided
+    static constexpr int DEFAULT_CELL_WIDTH = 16;
+    static constexpr int DEFAULT_CELL_HEIGHT = 16;
 public:
     UIGrid();
     //UIGrid(int, int, IndexTexture*, float, float, float, float);