Calculate the mouse focus inverse transform only when needed

[Sauermann]

Revival of #49158
Supersedes #49158
fix #69593
Co-authored-by: Marcel Admiraal madmiraal@users.noreply.github.com

madmiraal did the heavy lifting, I just fixed merge conflicts.

Orignal description:

Currently, gui.focus_inv_xform is used to determine the location of an event within a Control. Viewport::_gui_find_control() has the side-effect of updating gui.focus_inv_xform. This would be fine if _gui_find_control() was just used to assign gui.mouse_focus, but it's not. The result is, when a mouse button release event happens, sometimes gui.focus_inv_xform does not provide the location of the event inside the Control where the mouse button press event happened.

Instead of calculating the mouse_focus's Control's inverse transform every time _gui_find_control() is called and storing the result for later use, this PR calculates the required inverse transform only when it is needed. This will not only prevent the wrong inverse transform sometimes being used, but should result in better performance too.

Edited 2023-01-09: Fix merge conflict

[groud]

This seems to solve the problem correctly, as my own investigations lead to the same conclusion that focus_inv_xform had sometimes wrong values.

As the matrix multiplication to compute the transform should not be a very heavy processing, I don't think it a problem to remove this "optimization" and compute the value on demand.

[akien-mga]

Thanks!