Now scrollbars correctly hide and show, making space for the actual contents
in a region. It solves several old hacks, and puts view2d code a bit more
back in control as well.
Implementation notes:
- The view2d mask feature is working again
- The #define V2D_SCROLL_HORIZONTAL_HIDE means: "turn on hiding".
- Code for UI_view2d_region_reinit() is enforcing better standard view settings
But... two hack/patches needed to be added:
- Region panel drawing: if after generating the button panels it appears
a scroller hides or reveils, it calls all the generating code again.
(a simple scale doesn't work due to rounding differences in layout code)
- View2d code that maps 'tot' and 'cur' rects: if this code detects that
the mask changes, it calcs the map code again.
Also a bugfix (issue in 2.65)
- The left/bottom area split widget was drawing 1 pixel too large sometimes,
leaving bad trails on moving area dividers.
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