Blender Git Loki

Fix crash when changing space type to 3D space when having multiple windows

it is possible that different windows shares scene but displays different
layers. And it's also possible that different areas in the same window will
show different layers.

First case was violated in `dag_current_scene_layers()` which only checked
scene layers only once and if multiple windows shares the same scene only
one window was handled. Now made it so layers from all windows will be
squashed together into a single `DagSceneLayer`. This mainly solves issue
with `DAG_on_visible_update()` which didn't work reliable with multiple
open windows.

Second case required call of `DAG_on_visible_update()` when changing space
are type.

This commit slows things a bit actually because `dag_current_scene_layers()`
is actually called on every main WM loop iteration. It is possible to speed
some logic up perhaps. Not sure it's so much critical to do now because there
are unlikely to be more than few windows open anyway.

Will rather think of skipping all that flushing things if no objects are
tagged for update actually.