Merge branch 'master' into viewport (inc Vulkan resize fix merge #1042)

Style: CloseButton() now display a proper cross. Using Button colors for background. Removed ImGuiCol_CloseButton, ImGuiCol_CloseButtonActive, ImGuiCol_CloseButtonHovered as the closing cross uses regular button colors now. (#707)

Internals: Columns renaming fields

Viewport, DPI: Some early work on per-viewport DPI support. At the moment the easiest way is to replace fonts during the ChangedViewport callback, but down the line we should aim at handling some of it at ImFont level. (#1542, #1676)

Examples: DPI: Portable DPI related helpers in the _Win32 examples. Using one in examples's main.cpp, the GetDpiScale functions are not wired anywhere for now. (#1542, #1676)

Viewport: Fix dropping back viewport not being moved to the front of the window list. (Now that viewport are more tighly coupled to a single root window) (#1542)

Viewport, DPI: Don't activate current window until a viewport has been selected. At this point we'll have the proper size/scale for the current DPI + fix viewport callback when appending to an existing window more than once a frame. (#1542, #1676)

Viewport, DPI: Select viewport before locking style sizes and before handling double-click-on-title-bar to collapse. (#1542, #1676)

Merge branch 'master' into viewport

Viewport,Platform: Added GetWindowDpiScale() platform interface, changes of scale are reflected by positioning and resizing windows in their given viewport. (#1542)

Updated Changelog following merge of #1619 + fixed entry added to wrong version Changelog in 1ef1acb

Viewport: Coupling non-main viewport to one window. Viewport name derived from the window. Added ImGuiConfigFlags_PlatformNoTaskBar, off by default (aka re-established task-bars by default for now). (#1542)

Merge branch 'master' into font_offset

Comments, Changelog

Viewport: Disable extra viewports from hosting other windows. (#1542)

Fixed warnings.

Viewport: Reorganized viewport enable flags. Both user + platform + renderer need to enable a flag. (#1542)

Viewport: Modal display their darkneing/whitening layer over all viewports. (#1542)

Viewport: Various sanity fixes. Popup always inherit viewport from their parent for now. (#1542)

Merge branch 'master' into viewport ...

# Conflicts:
#	examples/imgui_impl_glfw.h
#	examples/opengl3_example/imgui_impl_glfw_gl3.cpp
#	examples/sdl_opengl3_example/imgui_impl_sdl_gl3.cpp
#	examples/sdl_opengl3_example/imgui_impl_sdl_gl3.h
#	imgui.cpp

Render: Removed the code that disable render if style.Alpha is 0.0f, it is both incorrect (as style Alpha can be modified mid-frame), not really necessary (just tested that full Alpha = 0 will lead to empty draw lists!) and misleading (bulk of the work was already done).

Viewport: Tracking current viewport we are appending to + added callback in PlatformInterface for DPI purpose (WIP). (#1542)

Fixed type cast warning.

Comments, FAQ update.

Columns: Fixed destructive small resize. (#1656). The OffsetNorm clamp introduced by #913 was ok as it didn't write back into the storage, which #1499 started doing making it destructive. Right now I don't think the clamp is needed at all. It had uses (eg: hide the issue fixed by bf7481e).

Columns: Extent stop at the right-most clipped pixel. The right-most column might appear a little wider but it's usable space matches the others. (#125). +9 Internal: Store InnerClipRect.

Columns: Clamping MaxX above MinX. This shouldn't have much of a benefit / affect, but the internal values are more sane this way. (#125)

Columns: Adding per-window display in Metrics. Tidying up, removing old code/comments. (#125, #1499, #1656)

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)