SDL support, continue
SDL support: UI image is drawn ok
SDL: mouse is working
SDL2 backend - keydown/keyup events support
SDL2: text input is working
investigating memory error in runtime shutdown
remove excessive logging
update DUB.JSON to use SDL
SDL2 platform: OpenGL support, part 1
OpenGL is working under SDL2
support SDL build w/o OpenGL
tweak button resources
update readme
example1 improvements: Buttons demo tab