Blender Git Loki

Fix T86331: Preferences menu inaccessible until window resize

The RGN_TYPE_EXECUTE region was zero height - with the "Load & Save"
button drawing outside of those bounds - so it didn't respond to events.

This is because the region started off this size and does not change
with simply adding the buttons. Although it has RGN_FLAG_DYNAMIC_SIZE,
delayed reinit of regions is only currently supported on headers.

This gives the execute region an initial (minimum) vertical size but
also makes the region **hidden** by default.

- Showing Prefs as an editor among others it will show the header
but not the execute region.

- Showing the Prefs in a popup window, hides the header region
and shows the execute region.

Ref D10636