Examples: SDL: Fix for versions olders than 2.0.6

Examples: SDL,GLFW,Vulkan: The Platform<>Renderer link is handled by SDL/GLFW platforms, both can compile without Vulkan headers, SDL+Vulkan is now on part with GLFW+Vulkan (aka broken the same way!). (#1542)

Examples: Using draw_data->DisplaySize, followup to c50198d. Fix Vulkan secondary viewport rendering. SDL+Vulkan: Matched changes. Fix vcprojs. (#1542, #1042)

Viewport: Fixed software mouse cursor from appearing in all multiple lists simultaneously. (#1542)

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

Viewport: Removed unnecessary function hoop.

Merge branch 'master' into viewport

Moved IM_NEW, IM_DELETE helpers to imgui.h for convenient use by back-ends and user code.

ImVector: Spacing, assert tweaks

Misc: Fix 111ea7a, GetStyleColorName() would assert, removed removed ImGuiCol defines (too much risk/complexity trying to silently ignore it).

Contributing: tweaks, mention screenshots more prominently.

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

Merge branch 'master' into viewport (inc Vulkan resize fix merge #1042)

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

Misc comments, todos + reintroduced removed ImGuiCol defines under !IMGUI_DISABLE_OBSOLETE_FUNCTIONS wrap.

Examples: DPI: Minor warning fix. (#1676)

Examples: Vulkan: Only resize swap chain and framebuffer once. (#1042)

Examples: Renamed glfw error callback for clarity.

Fixed resize window validation errors with removing frame prerender once and present last but one frame functionality. Frame rate is still similar.

Updated templates, added pull request template.

Style: CloseButton() now display a proper cross. Using Button colors for background. Removed ImGuiCol_CloseButton, ImGuiCol_CloseButtonActive, ImGuiCol_CloseButtonHovered as the closing cross uses regular button colors now. (#707)

Internals: Columns renaming fields

Viewport, DPI: Some early work on per-viewport DPI support. At the moment the easiest way is to replace fonts during the ChangedViewport callback, but down the line we should aim at handling some of it at ImFont level. (#1542, #1676)

Examples: DPI: Portable DPI related helpers in the _Win32 examples. Using one in examples's main.cpp, the GetDpiScale functions are not wired anywhere for now. (#1542, #1676)

Viewport: Fix dropping back viewport not being moved to the front of the window list. (Now that viewport are more tighly coupled to a single root window) (#1542)

Viewport, DPI: Don't activate current window until a viewport has been selected. At this point we'll have the proper size/scale for the current DPI + fix viewport callback when appending to an existing window more than once a frame. (#1542, #1676)

Viewport, DPI: Select viewport before locking style sizes and before handling double-click-on-title-bar to collapse. (#1542, #1676)

ImFont: Added FindGlyphNoFallback. Fixed MergeMode broken by 1ef1acb

Merge branch 'master' into viewport

Viewport,Platform: Added GetWindowDpiScale() platform interface, changes of scale are reflected by positioning and resizing windows in their given viewport. (#1542)