Fixed all issues found by vulkan debug report. Reasons for the major design changes are commented.

Examples: Vulkan: Disable IMGUI_VULKAN_DEBUG_REPORT by default. Update .bat files for newer VulkanSDK.

Vulkan backend: optionally enabling vulkan validation layers and debug report callback ...

Additional layer, extension and the callback itself are used/created when IMGUI_VULKAN_DEBUG_REPORT is defined. The callback reports seven (potential!) errors which will be fixed with another pull request.

Merge branch 'g_FrameIndex_not_used_fix' of https://github.com/ParticlePeter/imgui into ParticlePeter-g_FrameIndex_not_used_fix

Examples: Vulkan: windows batch file for 64-bits.

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

Examples: SDL2: Tweaked batch files for Win32.

Merge pull request #1080 from SaschaWillems/master ...

Examples: Vulkan: Small changes to be in line with specs and fix validation layer errors/warnings

Examples: SDL2: Added build .bat files for win32.

Examples: SDL2+GL3: Fix blend func state save/restore (#1120)

Fix blend func state save/restore ...

AFAIK there's no such enum as `GL_BLEND_SRC` or `GL_BLEND_DST` in OpenGL3 and OpenGL4.

Maybe they're left over from previous versions? They don't even seem to exist in OpenGL 2.1. In fact checking they were deprecated in OpenGL 1.5 (they exist in 1.3)

Comments to redirect people to OpenGL3 examples (#1116)

Examples: GLFW+GL3, SDL+GL3: Fixed modifying bound Texture0 (#1087, #1088, #1116)

Clip negative scissor offsets to zero

Set required depth member for buffer image copy

typo, #elif instead of #else, fixed

Vulkan backend: g_FrameIndex not used fix -> huge perf gain ...

Added a macro to switch between unlimited frame rate (VK_PRESENT_MODE_IMMEDIATE_KHR) and limited to 60 fps (VK_PRESENT_MODE_FIFO_KHR). Only the latter mode is guaranteed to be available, but the former one most likely is.

Fix scissor offset being negative

Fix new Vulkan validation warnings

Examples: Speculative fix for OSX Makefile to make Travis happy (re #812)

Examples: DirectX9: Explicitely setting viewport to match that other examples are doing (#937)

Examples: Vulkan: Coding style fixes (#910)

Vulkan example: Proper surface format search.

Vulkan example: Fix gamma for some implementations.

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

Examples: DirectX9-10-11: Only call Windows' SetCursor(NULL) when io.MouseDrawCursor is set (#585, #909)

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

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

Examples: GL3: Comments about gl3w (#880)

Fixed overzealous GCC warnings (#883) + partly undo 8553bab