Mask viewport split dividers under floating windows

The divider hit-zones are raw screen rects; gate them on
IsWindowHovered(ChildWindows) so a window stacked over the viewport
(e.g. the Material Editor) doesn't highlight or start a resize drag
through it. In-progress drags still track until release.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitignore

@@ -78,3 +78,4 @@ imgui.ini
 
 # OCIO config LUT files — downloaded automatically at CMake configure time
 resources/OpenColorIO-Configs/
+resources/hdri/