Viewport: Obsoleted io.DisplayPos (which was a Viewport branch thing), added ImDrawData::DisplayPos, DisplaySize (#1542) wip

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)

Viewport: Changed signature of ResizeViewport to be consistent with other similar functions.

Viewport: Reorganized viewport enable flags. Both user + platform + renderer need to enable a flag. (#1542)

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: 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: 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

Merge branch 'master' into examples_refactor

Examples: Vulkan: Added support for io.DisplayPos.

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