[nostalgia/core/studio] Add ability to select pixels for copy/paste

src/nostalgia/core/gfx.hpp

@@ -109,6 +109,10 @@ struct TileSheet {
 			return *this;
 		}
 
+		auto idx(const geo::Point &pt) const noexcept {
+			return ptToIdx(pt, columns);
+		}
+
 		/**
 		 * Reads all pixels of this sheet or its children into the given pixel list
 		 * @param pixels