Merge branch 'viewport' into docking ...

# Conflicts:
#	examples/example_allegro5/main.cpp
#	examples/example_marmalade/main.cpp

Merge branch 'master' into viewport + comments ...

# Conflicts:
#	examples/imgui_impl_opengl3.cpp
#	imgui.cpp

Nav: Fixed an assert in certain circumstance (mostly when using popups) when mouse positions stop being valid. (#2168) + adding a else block to make NavCalcPreferredRefPos() more explicit.

Internals: DragBehavior: Added support for ImGuiDragFlags_Vertical to implement a vertical drag widget (no frontend function provided).

Merge branch 'viewport' into docking ...

# Conflicts:
#	imgui_demo.cpp

Merge branch 'master' into viewport ...

# Conflicts:
#	imgui.cpp

Comments in demo and opengl code + Internals: Added HoveredIdNotActiveTimer tracking hovering time unless the item is active, which is a commonly useful pattern.

Merge branch 'viewport' into docking ...

# Conflicts:
#	imgui.cpp
#	imgui.h

Viewport: Exposed GetOverlayDrawList(ImGuiViewport* viewport) in public API. (#1542, #1660, #1878, etc.)

Merge branch 'master' into viewport ...

# Conflicts:
#	examples/imgui_impl_sdl.cpp
#	imgui.cpp

Merge branch 'master' into docking ...

# Conflicts:
#	examples/imgui_impl_sdl.cpp
#	imgui.cpp
#	imgui_internal.h

Shutdown: Unlock font atlas before destroying context, so we can destroy a context between NewFrame and EndFrame if we wait (facilitate main loop structures). Internals: GetWindowScrollMaxX(), GetWindowScrollMaxY()

TextUnformatted: Using memchr(), fixed not properly testing for text_end bound + comments. ...

Internals: Added ImStreolRange() + used in LogRenderedText() + comments.

Comments + internal using Tab Stop terminology (ImGuiItemFlags_NoTabStop instead of !ImGuiItemFlags_AllowKeyboardFocus)

Docking: Kept ImGuiDockNodeFlags_PassthruDockspace and removed ImGuiDockNodeFlags_PassthruInEmptyNodes, ImGuiDockNodeFlags_RenderWindowBg. Doesn't include ImGuiDockNodeFlags_NoDockingInCentralNode. (#2109)

Docking: Removed context parameter from internal DockBuilderXXX api at it is expected we transition it to be public facing. (#2109)

Docking: Added ImGuiDockNodeFlags_PassthruDockspace mode (subdivided in three flags: ImGuiDockNodeFlags_NoDockingInCentralNode, ImGuiDockNodeFlags_PassthruInEmptyNodes, ImGuiDockNodeFlags_RenderWindowBg). Added internal facility for register a rectangular hit-test hole in window. Updated DockSpace demo accordingly. (#2109)

Docking: Renamed "DocRoot/DocumentRoot" to "CentralNode", more self explanatory. Moved Splitter update higher up in DockNodeUpdate() + minor misc tweak. (#2109)

Docking: Fixed DockSpace() child window displaying a scrollbar behind the node backgrounds. Rename ImGuiCol_DockingBg to ImGuiCol_DockingEmptyBg. Added ImGuiDockNode::IsLeaftNode(). (#2109)

Docking: Fixed dock node with single window accidental stealing of non-owned host viewport (leading to general havoc).

Docking: Added DockBuilderCopyDockspace() wip. Added DockBuilderCopyWindowSettings(), renamed DockBuilderForkNode() to DockBuilderCopyNode().

Added FindOrCreateWindowSettings() internal helper.

ImPool: Added Reserve() helper function.

Docking: Fixed tab order restoring (follow up to 67be485e, broken in 455dc6e2)

Docking: Added DockBuilderForkNode() primitive for usage for user code to fork/copy entire layouts. This will probably need to be added to as I find ways to migrate patterns from user code to the library. Added IMGUI_DEBUG_DOCKING_INI helper to facilitate debugging. (+1 squashed commits)

Docking: Reworked SetNextWindowUserType() as SetNextWindowDockFamily(), allowing multiple scene to be tagged with their scene id and not receive tabs from other scene + persistent storage for family id to allow for docking tree fork/remap.

Docking: Added DockBuilderGetNode() wrapper for consistency.

Docking: Added DockBuilderRemoveNode() + various tweaks. Fixed dragging/undocking dock node from CollapseButton.

Docking: Fix for undocking node where the dockspace node is attached to an immovable parent window. + Tweak .ini output.

Docking: Some DockBuilder functions are applied on settings data if windows are not present. Added DockBuilderCreateNode which needs a size else if we can't split properly. DockNodeTreeSplit() doesn't clamp SizeRef. (+1 squashed commits)