WIP: use viewport to allow sizes of scroll to extend further than renderable screen

src/size.zig

@@ -1,10 +1,10 @@
-pub const Size = @This();
+pub const Size = packed struct {
+    anchor: Position = .{},
+    cols: u16 = 0,
+    rows: u16 = 0,
+};
 
-pub const Position = struct {
+pub const Position = packed struct {
     col: u16 = 0,
     row: u16 = 0,
 };
-
-anchor: Position = .{},
-cols: u16,
-rows: u16,