Examples: Vulkan: Fix for empty draw data (init time) + fixed warning.

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.

Merge branch 'context' ...

# Conflicts:
#	examples/allegro5_example/main.cpp
#	examples/directx10_example/main.cpp
#	examples/directx11_example/main.cpp
#	examples/directx9_example/main.cpp
#	examples/marmalade_example/main.cpp
#	examples/opengl2_example/main.cpp
#	examples/opengl3_example/main.cpp
#	examples/sdl_opengl2_example/main.cpp
#	examples/sdl_opengl3_example/main.cpp
#	examples/vulkan_example/main.cpp

Added ImGuiKey_Space, mapped in every examples. Will be required for navigation. (#787)

Examples: Organized header files to make available features more visible.

Merge branch 'master' into context

Examples: Using #include "imgui.h" instead of <imgui.h> appear correct-er as per standard? xcode+clang are very nitpicky there.

Examples: Vulkan: FIxed warnings in 32-bit modes (vulkan uses VkDeviceSize which is always 64-bit long)

Examples: Updated for reorganized context functions. Calling CreateContext(), DestroyContext() in example code. Removed Shutdown() from binding code. (#1565, #586, #992, #1007, #1558)

Examples: Simplified mouse wheel handling. (#1463)

Examples: Allegro, Apple, DirectX9/10/11, Glfw+Vulkan :Added support for horizontal mouse wheel. (#1463)

Added ImGuiKey_Insert for future use. Setup in all example bindings. (#1541)

Examples: Comments about invalid mouse pos.

Examples: Added a bunch of comments/referencs related to io.WantCaptureMouse, io.WantCaptureKeyboard (#1262, #1237, #1219, #635, #1058, #1051, #912, #533, #703, #446, #459, #364, #213, #52, and more)

IO: io.MousePos needs to be set to ImVec2(-FLT_MAX,-FLT_MAX) when mouse is unavailable/missing. Previously ImVec2(-1,-1) was enough but we'll now accept negative mouse coordinates.

Examples: Vulkan: Fixed an extra validation (tested on Windows with VulkanSDK 1.0.46.0) (#1042)

Merge branch 'vk_small_fix' of https://github.com/martty/imgui into martty-vk_small_fix

Clip negative scissor offsets to zero

Set required depth member for buffer image copy

Fix scissor offset being negative

Fix new Vulkan validation warnings

Vulkan example: Fix validation layer warnings and errors and redeclare gl_PerVertex.

Examples: Vulkan: Shallow stylistic changes (following #879)

Add location decorators & change to use structs as in/out in glsl, update embedded spv (produced with glslangValidator -x)

Null the released resources

Added a void* user_data parameter to Clipboard function handlers. (#875)

Examples: Accessing ImVector fields directly, feel less stl-ey and fix renderer side assert when render list only contains a callback (#810)

Fix Vulkan example for use when a depth buffer is active.

tabs to space, remove braces