Backends: GLFW: Handle spurious leave/enter sequences apparently submitted by X11/GLFW combo. (#4984)

# Conflicts:
#	backends/imgui_impl_opengl3.cpp
#	docs/CHANGELOG.txt

Backends: SDL: Added SDL_Renderer* parameter to ImGui_ImplSDL2_InitForSDLRenderer(). Use SDL_GetRendererOutputSize() instead of SDL_GL_GetDrawableSize() when bound to a SDL_Renderer. (#4927)

This is (kind of) an OpenGL-only function, which should be avoided when SDL2 isn't using OpenGL.
The only alternative that is recommended is SDL_GetRendererOutputSize, which limits this fix to the SDL_Renderer backend. Still, I think it's better than nothing.
I say that SDL_GL_GetDrawableSize is "kind of" OpenGL-only because it does technically work even when SDL2 isn't using OpenGL.
It's just that it becomes a shim to SDL_GetWindowSize, which is not suitable for high-DPI usage because it reflects the size of the window in screen coordinates, not actual pixels, so it really should be avoided when not using OpenGL.

SDL_PIXELFORMAT_RGBA32 is intended for byte arrays where the channels
are specifically in the RGBA order. However, this is not what
GetTexDataAsRGBA32 does: rather, it constructs an array where each
pixel is an unsigned int in ABGR order. On little-endian platforms,
this does indeed result in an RGBA byte order, however, on big-endian
platforms, this results in an ABGR byte order, which does not match
the PIXELFORMAT enum.

What should be used is the SDL_PIXELFORMAT_ABGR8888 enum, which
specifies that the pixels are in native-endian ABGR, which they are.

Builds have failed on Xcode versions that do not yet support `@available` or do not have new APIs (that are unavailable on target OS version) defined at all. Using true build time version checks fixes these issues.

Backends: OSX, Android: Replaced AddKeyModsEvent() added by 790132a6 in favor of unified key enum system. (#4921, #4858)

(BREAKING) Replaced AddKeyModsEvent() added by 790132a6 in favor of unified key enum system. Backends: update most. (#4921, #4858)

Sorry this is an unusual breaking but since we are WIP it is a good time to make a correction.

Rendering: defer responsability of calling _PopUnusedDrawCmd() later so RenderDimmedBackgrounds()/RenderMouseCursor() don't need to deal with the side-effects (#4857, #4317)

Don't merge ImDrawCmd which have had their IdxOffset changed to not be sequential. Fixed CTRL+Tab into an empty window causing artefacts on the highlight rectangle due to bad reordering on ImDrawCmd.

This is bit of a weird edge case adding weight to ImDrawCmd merging, if we could rework the mess in RenderDimmedBackgroundBehindWindow() we may be able to undo some of that.

Shallow alignment of ImGuiKey stuff + moving some in internals. Internals: add offset to ImBitArray<>, simpify ActiveIdUsingKeyInputMask.

Backends: GLFW: fix ImGui_ImplGlfw_TranslateUntranslatedKey() for grave accents. (#456, #2625) + fix preceeding commit.

Amend 100ede57 + amend b17b2fb7

Popups: Fixed an issue when reopening a same popup multiple times would offset them by 1 pixel on the right. (#4936)

Passing explicit ImGuiPopupFlags_MouseButtonRight to OpenPopupOnItemClick() calls somehow document the unusual (due to legacy) default value.

Demo: draw a section of keyboard in "Inputs > Keyboard, Gamepad & Navigation state" to visualize keys.

Backends: OSX: Add full gamepad support using io.AddKeyEvent(), io.AddKeyAnalogEvent(), stopped writing to io.NavInputs[]. (#4858, #787)

Backends: SDL: Add full gamepad support using io.AddKeyEvent(), io.AddKeyAnalogEvent(), stopped writing to io.NavInputs[]. (#4858, #787)

Backends: GLFW: Add full gamepad support using io.AddKeyEvent(), io.AddKeyAnalogEvent(), stopped writing to io.NavInputs[]. (#4858, #787)

Backends: Win32: Add full gamepad support using io.AddKeyEvent(), io.AddKeyAnalogEvent(), stopped writing to io.NavInputs[]. (#4858, #787)

Backends: Vulkan: Added support for ImTextureID as VkDescriptorSet, add ImGui_ImplVulkan_AddTexture(). (#914)