Retain Win32 entry state and trim canvas/platform seams

src/legacy_canvas_draw_merge_services.h

@@ -239,6 +239,17 @@ struct LegacyCanvasDrawMergePostDrawExecution {
     std::function<void()> draw_current_modes;
 };
 
+template <typename GridT>
+[[nodiscard]] inline auto make_legacy_canvas_draw_merge_heightmap_draw(
+    GridT* grid,
+    const glm::mat4& proj,
+    const glm::mat4& camera)
+{
+    return [grid, proj, camera] {
+        grid->draw_heightmap(proj, camera, false);
+    };
+}
+
 struct LegacyCanvasDrawMergeSmaskFacesExecution {
     std::function<void()> set_active_texture_unit;
     std::function<void()> enable_blend;