Merge branch 'master' into viewport ...

# Conflicts:
#	examples/allegro5_example/main.cpp
#	examples/directx10_example/main.cpp
#	examples/directx11_example/main.cpp
#	examples/directx12_example/main.cpp
#	examples/directx9_example/main.cpp
#	examples/imgui_impl_dx10.cpp
#	examples/imgui_impl_dx11.cpp
#	examples/imgui_impl_dx12.cpp
#	examples/imgui_impl_dx9.cpp
#	examples/imgui_impl_glfw.h
#	examples/imgui_impl_vulkan.cpp
#	examples/opengl2_example/imgui_impl_glfw_gl2.cpp
#	examples/opengl2_example/main.cpp
#	examples/opengl3_example/imgui_impl_glfw_gl3.cpp
#	examples/opengl3_example/main.cpp
#	examples/sdl_opengl2_example/imgui_impl_sdl_gl2.cpp
#	examples/sdl_opengl2_example/main.cpp
#	examples/sdl_opengl3_example/imgui_impl_sdl_gl3.cpp
#	examples/sdl_opengl3_example/main.cpp
#	examples/vulkan_example/main.cpp
#	imgui.cpp
#	imgui.h

Viewport: Give a chance to platform and renderer to actually destroy their data. Otherwise the regular ImplXXX_Shutdown + following by DestroyContext() order fails to fullifl this. (#1542)

Renamed ImGuiStyleVar_Count_ to ImGuiStyleVar_COUNT for consistency with other enums. Added

Added ArrowButton(). Exposed ImGuiDir.

BeginCombo(): Added ImGuiComboFlags_NoArrowButton and ImGuiComboFlags_NoPreview flags + hover color matches drag and sliders.

Merge branch 'patch-1' of https://github.com/stfx/imgui into stfx-patch-1

Combo: Arrow button isn't displayed over frame color, so color is consistent with other button + the button doesn't have inner rounding.

Internal: renamed RenderTriangle() to RenderArrow().

Viewport: Replaced UpdatePlatformWindows/RenderPlatformWindows by RenderAdditionalViewports(). The update is always called in EndFrame(). (#1542)

Viewport: When enabled we assert against using RenderDrawListFn to make user more aware of rendering order. (#1542)

Viewport: Always be lenient with invalid mouse position during dragging, since they are provided by back-end and technically acceptable inputs + comments. (#1542)

Nav: Added links to PNG/PSD files for PS4 and Switch. Fixed suggested Joy-con mapping as per typical Nintendo-style mapping. (#787)

Internal: Moved IM_NEWLINE helper to imgui_internal.h

Renamed ImGuiNavFlags io.NavFlags to ImGuiConfigFlags io.ConfigFlags. (#787)

Renamed ImGuiNavFlags io.NavFlags to ImGuiConfigFlags io.ConfigFlags. (#787)

Viewport: Initial viewport branch+ platform api commit (squashed from previous commits, still very WIP) (#1542)

Metrics: Added option to show begin order, useful for debugging.

Internal: Minor renaming.

Internals: Moving window: test mouse validity while translating window + using SetWindowPos().

Removed DisplayVisibleMin/DisplayVisibleMax facility (as part of # 1542). Misc comments.

Clearing next window data at the end of Begin() more reliably. Misc comments.

Internals: Settings: Added ReadCloseFn to allow handlers to sanitize data on a per-entry basis.

Merge branch 'master' into examples_refactor ...

# Conflicts:
#	examples/opengl3_example/imgui_impl_glfw_gl3.cpp
#	examples/sdl_opengl3_example/imgui_impl_sdl_gl3.cpp

ImRect: Removed misleading IsFinite() function used by some Nav code.

Compacted some old Breaking Changes notes (Pre July 2015 stuff)

Changelog: Added all change infos since 1.53 up to the current version.

TreeNode: Renamed Beta ImGuiTreeNodeFlags_NavCloseFromChild to ImGuiTreeNodeFlags_NavLeftJumpsBackHere. (#1079)

Merge branch 'master' into examples_refactor

Merge branch 'master' of https://github.com/podgorskiy/imgui into podgorskiy-master

A tiny optimization to ImLineClosestPoint. Removed unnecessary sqrtf call. ...

ab_lenSqr -> ab_len_sqr
Moved line where ab_len_sqr is computed after the first return