Readme: fix appveyor badge

Updated engine initialization

Updated ImGui UWP event handler

GLFW Demo: fixed image format mismatch Vulkan validation warning

GLTF Viewer: use sRGB textures

SampleApp: make the sample release swap chain buffers when toggling f…

…ull screen mode

Tutorial 28: code style changes; enabled OpenXR on GHA/Win32

Tutorial 28: code style changes; enabled OpenXR on GHA/Win32

Web: Fixed behaviour of warning box

GLTFViewer and USDViewer: use FILE_DIALOG_SUPPORTED macros