Viewports, DPI: Renamed config flags. (#1542)

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

Viewport: Removed back-end renderer code that implied we need to clear with WindowBg color. Added NoRendererClear flag. (#1542, #1042)

Examples, Viewport: Vulkan: Experiment (broken) multi-viewport support, merging code from ParticlePeter branches. (#1542, #1042)

Examples: Vulkan: Passing Queue, QueueFamily to binding. Fixed scissor (fixed in master). ImGui_ImplVulkanH_DestroyWindowData() waits for device to be idle. (#1042)

Examples: Vulkan: Further refactor.

Examples: Vulkan: Moved CreateOrResizeWindowData func to imgui_impl_vulkan.h

Examples: Vulkan: Moved main.cpp data structures to imgui_impl_vulkan.h as shared helpers for the benefit of both the example and the platform/viewport renderer interface.

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

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.

Examples: Titles/comments in headers

Examples: Vulkan: Extracted into imgui_impl_vulkan.*, reused imgui_impl_glfw* files.