Added support for SDL and GLFW

[leokolln]

Implemented the equivalent of --with-sdl and --with-glfw.

Usage:

cmake -DBGFX_WITH_SDL=TRUE ..

or

cmake -DBGFX_WITH_GLFW=TRUE ..

Only tested on Ubuntu 18.04, relevant packages versions:

Package: libsdl2-dev
Version: 2.0.8+dfsg1-1ubuntu1.18.04.1

Package: libglfw3-dev
Version: 3.2.1-1

Package: cmake
Version: 3.10.2-1ubuntu2

[JoshuaBrookover]

I'm not able to test Windows right now but I'm not seeing any reason why the change list should cause issues, especially since its opt-in.

Tested on OSX. GLFW works great, SDL2 has issues as you suspected. Unfortunately I don't think there's anything I can do about SDL2 when installed as a framework. BGFX's entry includes SDL2 as <SDL2/SDL.h> but in the OSX framework the "SDL2" folder doesn't exist.

People will have to manually fix this themselves if they plan to use SDL2, which should be easy enough, so I'll go ahead and merge in. Thanks!