use nested map for SDF names to save stack space.

Update semantics of the SDF array to (row, column) form to match
the picker space

Fix #674

src/scenedit/scen.sdfpicker.hpp

@@ -16,6 +16,8 @@ class cTilemap;
 
 class cStuffDonePicker {
 	cDialog dlog;
+	// Note: x and y in initial_sdf and chosen_sdf are actually (c, r) and must be flipped when
+	// indexing SDFs or their names in the scenario
 	location initial_sdf, chosen_sdf, viewport;
 	cTilemap* grid;
 	cTilemap* row_labels;