Examples: OpenGL2: Added multi-viewport support in the OpenGL2 back-end. (#1542) + Metrics: Fix undisplayed flag.

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

IO: Added ImGuiConfigFlags_NoSetMouseCursors. Added ImGuiBackendFlags_HasMouseCursors, ImGuiBackendFlags_HasSetMousePos. (#787, #1495, #1202)

Added ImGuiBackendFlags for backend to expose its capabilities. Added ImGuiBackendFlags_HasGamepad. (#787, #1542)

IO: Renamed io.WantMoveMouse to io.WantSetMousePos (was added in 1.52, _not_ used by core and only honored by some binding ahead of merging the Nav branch) + internal renaming (#787)

imconfig: Added IMGUI_STB_TRUETYPE_FILENAME, IMGUI_STB_RECT_PACK_FILENAME + documenting IMGUI_DISABLE_STB_XXX flags and hiding IMGUI_STB_NAMESPACE which is misleading. ...

Fixed typos in todo list.

Merge branch 'master' into viewport ...

# Conflicts:
#	examples/sdl_opengl2_example/main.cpp
#	examples/sdl_opengl3_example/main.cpp
#	examples/vulkan_example/main.cpp

Examples: Renamed applications to emphasis on use of GLFW as a platform framework + minor local tweaks

ImVector: Fixed insert() helper using the = operator (followup to be consistent with 4186c2c)

Viewport: Changelog + Internals: Renamed viewport mouse fields for consistency (going to add more). (#1542)

Fixed enum typo.

Viewport: Added extra Metrics and debug features.

Viewport, Platform: Update the contents of GetPlatformData() in EndFrame() so back-end can access it even if UpdatePlatformWindows hasn't been called. (#1542)

Examples: Vulkan: Fix missing subpass dependency ...

Without a dependency between pWaitDstStageMask (COLOR_ATTACHMENT_OUTPUT)
and the render-pass, the UNDEFINED -> COLOR_ATTACHMENT_OPTIMAL transition
might happen before the image is ready to be used.

Examples: Vulkan: Using VK_PRESENT_MODE_MAILBOX_KHR. Disable IMGUI_UNLIMITED_FRAME_RATE by default. Fixed warnings. (#1042)

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

Demo: Added more complete Combo demo. (#1658)

Nav: InputText: Allow editing text input fields with NavActivate (space). Not sure about that, on one hand it feels more consistent but you can't finish the editing with space to double space (activate + input space) could feel inconsistent. (#787)

Nav: Track nav input source more generally (gamepad vs keyboard) (#787) + update todos and demo tweaks

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)

Merge branch 'master' into viewport ...

# Conflicts:
#	imgui.h

imgui.h: Various comments and tweaks.

imgui.h: Moved columns lower in the file. Various spacing changes and minor comments.

Merge branch 'master' into viewport ...

# Conflicts:
#	imgui.h

ImVector: Added copy/assign operators as helpers to ease cloning of data.

ImDrawList: Added Clone() helper function.

ImVector: resize() / push_back() use memcpy() as we are not supposed to be constructor/destructor/operator aware at all. (If we do need to rework ImVector). In turn this will allow us to define assign/copy operators.

ImVector: Using value_type consistently. Spacing.

Update TODO list, minor fix in Readme, comments, Clang -Weverything warning fix in imgui_draw.cpp under Windows.