Viewports, DPI: Renamed config flags. (#1542)

Examples, Platform, Viewport: Fixed inconsistent window ownership issues. Added comments. Made Win32/SDL back-ends track ownership.

Merge branch 'master' into viewport ...

# Conflicts:
#	examples/imgui_impl_dx10.cpp
#	examples/imgui_impl_dx11.cpp
#	examples/imgui_impl_dx12.cpp
#	examples/imgui_impl_dx9.cpp
#	examples/opengl2_example/imgui_impl_glfw_gl2.cpp
#	examples/opengl3_example/imgui_impl_glfw_gl3.cpp
#	examples/sdl_opengl2_example/imgui_impl_sdl_gl2.cpp
#	examples/sdl_opengl3_example/imgui_impl_sdl_gl3.cpp
#	examples/vulkan_example/imgui_impl_glfw_vulkan.cpp
#	imgui.h

Viewport, Platform: Added void* to render/swap buffer functions. Comments. (#1542, #1042)

Viewport, Examples: Renamed types used by examples. (#1542)

Viewport, Platform: Refactored platform interface. Removed need to use imgui_internal.h in backends. Split viewport into public facing ImGuiViewport and internal structure. Exposing enough data to provide custom tweaked renderers. Renamed handlers, fixed lots of inconsistencies. (#1542, #1042)

Viewport: Removed back-end renderer code that implied we need to clear with WindowBg color. Added NoRendererClear flag. (#1542, #1042)

Viewport: Obsoleted io.DisplayPos (which was a Viewport branch thing), added ImDrawData::DisplayPos, DisplaySize (#1542) wip

Viewport: Changed signature of ResizeViewport to be consistent with other similar functions.

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

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)

Examples: DX10: Multi viewport/platform support (based on DX11 version). (#1542)

Examples: DX11 + Win32: Initial attempt at implementing the viewport/platform api. (WIP/test API) (#1542)

Examples: Titles/comments in headers

Merge branch 'master' into examples_refactor

Examples: DirectX10, DirectX11, OpenGL2, OpenGL3: Added support for io.DisplayPos.

Examples: DirectX10: Reusing imgui_impl_win32, removed that code from imgui_impl_dx10

Examples: DirectX11: Extracted imgui_impl_dx11.* instead imgui_impl_dx11 and imgui_impl_win32