Scroll to active widget when switching active parent

[RobLoach]

nuklear_console.h:651 carries a TODO: Fix the scroll on the new window, since it may not be centered on the active widget. This is the same family of bug being reported externally in #170 ("Trying to force scroll of a page, but I'm having trouble") — user calls nk_console_set_active_parent + nk_console_set_active_widget but scroll never lands on the target.

Suggested Implementation

In nk_console_set_active_parent, after nk_window_set_scroll(top->ctx, 0, 0), also set data->scroll_to_widget = data->active_widget so the next render pass scrolls to the focused widget instead of pinning to the top.

QA

• Manual repro from Trying to force scroll of a page, but I'm having trouble. #170 stops misbehaving.
• New test: build a parent with N rows, set active widget = last row, set active parent, render two frames, assert window scroll Y > 0.