gui: fix zoom-to-mouse
Plater disappeared when zoom-to-mouse enabled and user tries to zoom
with a side view enabled (Ctrl+<3..6>).
Mouse/screen center position estimation in 3D is made by calculating
intersection of a mouse vector which comes 'through' the screen with an
XY plane. However, when screen is parallel to Z axis, intersection point
located at infinite which results in infinite camera translation vector.
This change switches mouse position estimation to use of projection
matrix like its done for camera panning.
9dad4f6433