render: reposition cursor when moving to new row

Explicitly reposition the cursor when moving to a new row to enable
terminals that support text selection to _not_ select the entire screen,
since the entire screen is wrapped.

src/vaxis.zig

@@ -309,6 +309,7 @@ pub fn Vaxis(comptime T: type) type {
                 if (col >= self.screen.width) {
                     row += 1;
                     col = 0;
+                    reposition = true;
                 }
                 // If cell is the same as our last frame, we don't need to do
                 // anything