Merge branch 'master' into drag_and_drop

Scrollbar flicker fix for menus/popups (fixes 4d00dd8 and 2df8fa9). Hopefully right this time.

Columns: Clipping columns borders on Y axis on CPU because some GPU drivers appears to be unhappy with triangle spanning large regions (not sure why tbh). (#125) ...

Demo: Columns: Adding a billion of extra lines and using clipper.

Drag and Drop: Added payload->IsPreview() to user can render their custom preview while handling overlapping drop targets. (#143)

Fixed build (3e06450 not meant for master branch, but ok)

Internals: Added ArrowButton() helper.

Drag and Drop: Added internal BeginDragDropTargetCustom() convenient to avoid submitting dummy ItemAdd. (#143)

Fixed scrollbar flickering on/off when uncollapsing a window (fixes 2df8fa9)

io.OptNoCursorBlink -> io.OptCursorBlink (#1427)

Added io.OptNoCursorBlink option to disable cursor blinking. (#1427). Renamed io.OSXBehaviors to io.OptMacOSXBehaviors. Should affect users as the compile-time default is usually enough. (#473, #650)

Clean g.WindowsById storage on shutdown.

Begin: Fixed appending into a child window with a second Begin() from a different window stack querying the wrong window for the window->Collapsed test.

ButtonBehavior: Fixed ImGuiButtonFlags_AllowOverlapMode to avoid temporarily activating widgets on click before they have been correctly double-hovered. (#319, #600)

Drag and Drop: Drop target rectangle goes out of clipping range (#143)

Drag and Drop: Allow NULL payload (since type only can be useful). (#143)

Fixed vertical scrollbar flickering/appearing, typically when manually resizing and using a pattern of filling available height (e.g. full sized BeginChild). THIS IS A GREAT FIX, this glitch was nasty and annoying (and yet somehow nobody reported it?). Hopefully haven't broken anything else...

Minor tweaks/comments. Note that the reordering the one subtraction caused subtle havoc before the patch in 9ac8820.

Fixed non-pixel aligned bounding box of window resize grip, / which triumphally led to any re-arrangement of operations inside the resize grip code outputting non-exact size_target values which led to unstable window position because clamping code uses size in a subtraction, etc etc. Lovely how a whole system can be made to act weird with a single bad input.

Drag and drop: Handle overlapping drag target priorities given their surface, which appears to make most sense for drag and drop operations.

Child window with MenuBar use regular WindowPadding.y so layout look consistent in child or in a regular window.

ButtonBehavior: Fixed ImGuiButtonFlags_NoHoldingActiveID from incorrectly setting ActiveIdClickOffset, which probably have no known effect, but it is more correct this way. (#1418)

LowerBound() minor tweaks

Internal: FindWindowByName() faster and doesn't touch every windows

Fixed auto-resize allocating too much space for scrollbar when SizeContents is bigger than maximum window size (fixes c0547d3) (#1417)

Begin: Simplified code and fixed a bug where appending into a window a second time (from a remote window) would incorrectly overwrite RootWindow with the current window in the stack. Our docking code uses this pattern.

Drag and Drop: Moved internal fields out of public sight. (#143)

Drag and Drop: Rework internal to allow overlapping targets (#143)

RenderTriangle() minor tweak to align Left/Right and Up/Down arrow extents visually

Drag and Drop: Made it legal to not call SetDragDropPayload() between BeginDragDropSource() and EndDragDropSource(). (#143)

Merge branch 'master' into drag_and_drop