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

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

Examples: Updated Makefile and batch files to work with new examples. Using lowercase "gl.h" as it seems this is what Linux wants.

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

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

Examples: DX12: Empty skeleton toward implementing the multi-viewport api for DirectX12. (WIP/test API) (#1542, #301)

Examples: DirectX12: Merged to new example format, imgui_impl_dx12.cpp contains the DX12 stuff, couple with imgui_impl_win32.cpp

Examples: DirectX12: Tweaked assertion to more accurately represent what it wants to say. (#301)

Examples: DirectX12: Fixed shutdown issue. (#301)

Examples: DirectX12: Merge, various styling tweaks, update for 1.60 wip, synchronized Win32 features with other examples. (#301)

Pass render target format in ImGui_ImplDX12_Init() instead of hard-coded.

Pass command list in using ImGui_ImplDX12_NewFrame() instead of ImGui_ImplDX12_Init()

Merge 'ocornut/master' into 'jdmo3/master'

Merge latest from ocornut/imgui

Examples: DirectX12: Add directx12_example