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)

Examples Refactor: GLFW: Explicit functions to init GLFW with OpenGL or Vulkan since we cannot read the api hints from glfw.

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)

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.

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

Renamed ImGuiStyleVar_Count_ to ImGuiStyleVar_COUNT for consistency with other enums. Added

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

Examples: Renamed glfw error callback so it's more clear what it is.

Merge branch 'examples_refactor' into viewport ...

# Conflicts:
#	examples/directx10_example/main.cpp
#	examples/directx11_example/main.cpp
#	examples/directx12_example/main.cpp
#	examples/opengl3_example/main.cpp
#	examples/sdl_opengl2_example/main.cpp
#	examples/sdl_opengl3_example/main.cpp
#	examples/sdl_vulkan_example/main.cpp
#	examples/vulkan_example/main.cpp
#	imgui.h

Do not store compiled objects outside the project tree.

Define SOURCES instead of OBJS in Makefiles. ...

OBJS is still deduced from SOURCES, but this change gives better control
over where the object files get actually written.

Use make variables $@ and $^ where appropriate.

Ensure `make clean` is idempotent. ...

Prior to this, `make clean` would fail if the project was not already
fully built, and a second invokation would always fail.

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

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

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

Examples: GLFW + OpenGL3: Initial attempt at implementing the viewport/platform api. (WIP/test API) (#1542)

Examples: Fix d749d49 missing the deletion code. (#1217)

Examples: OpenGL3: Create the VAO in the render function so the setup can more easily be used with multiple shared GL context. (#1217)

Examples: OpenGL: Cast call to glPolygonMode(). (#1628).

Examples: Mouse cursor handling comments/tweaks to homogenize. (#1495)

Examples: GLFW: Added support for mouse cursors (ImGui::GetMouseCursor() value and WM_SETCURSOR message handling). (#1495)

Examples: GLFW*: Renamed GLFW callbacks exposed in .h to not include GL2/GL3/Vulkan in their name.

Examples: GLFW+GL2: Moved glfw, gl2 files to root folder, split imgui_impl_glfw_gl2 into _opengl2 and reused imgui_impl_glfw

Examples: GLFW+GL3: Split imgui_impl_glfw_gl3 into imgui_impl_glfw and imgui_impl_gl3 (wip)

Examples: GLFW+GL2/GL3: Added glPixelStorei() calls borrowed from SDL examples.

Obsoleted the io.RenderDrawListsFn callback, you can call your graphics engine render function after ImGui::Render(). Use ImGui::GetDrawData() to retrieve the ImDrawData* to display..(#1599) ...

Examples: Updated examples.

Examples: Added Changelog to make updates easier.