Viewport: When enabled we assert against using RenderDrawListFn to make user more aware of rendering order. (#1542)

Examples: Vulkan: Moved variables into per-window data, refactored/tweaks to that the bug functions main.cpp for both SDL and GLFW examples match (they'll probably be moved to imgui_impl_vulkan as helpers).

Examples: Vulkan: Fix 7b968b0

Examples: Vulkan: Reduced duplicate code by skipping present on the first frame. Amend 201d589 by @ParticlePeter

Examples: Vulkan: Various tweak. Misc refactor into per-frame data. Duplicate buffer creation code moved to CreateOrResizeBuffer().

Examples: GLFW, Vulkan: GLFW binding viewport tweaks to supports Vulkan better (do not call SwapBuffer, share context etc.). Added DUMMY (empty) platform/viewport interface in the viewport code.

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

Examples: Vulkan: SDL: Fixed missing resize handler (not properly merged from #1367) + tweaks.

Examples: Vulkan: Moved code into shared helpers: ImGui_ImplVulkan_SelectSurfaceFormat, ImGui_ImplVulkan_SelectPresentMode.

Examples: Vulkan: Various tweaks to name variable more consistently like Vulkan + a few comments + a few imgui style code compaction.

Viewport: Always be lenient with invalid mouse position during dragging, since they are provided by back-end and technically acceptable inputs + comments. (#1542)

Examples: Vulkan: Debug report tweak + always enable in Debug build.

Examples: Fixed enabling IMGUI_VULKAN_DEBUG_REPORT which was broken during refactor because of duplicate 'extensions' local variable.

Allow user to override ImTextureId. (#1641)

Makefile: updated sdl_opengl3_example with all changes from #885.

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.

Nav: Added links to PNG/PSD files for PS4 and Switch. Fixed suggested Joy-con mapping as per typical Nintendo-style mapping. (#787)

Internal: Moved IM_NEWLINE helper to imgui_internal.h

Added ImGuiConfigFlags_IsSRGB, ImGuiConfigFlags_IsTouchScreen (strictly for user storage)

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

Examples: NULL pointers for consistency.

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

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

ImGui_ImplSDL2_Init() now takes a SDL GL context.