citra: Remove GLFW, Add SDL2
FindSDL2: Do not CACHE SDL2_* variables if library is not found
EmuWindow_SDL2: Set minimal client area at initialisation time
EmuWindow_SDL2: Corrections
EmuWindow_SDL2: Fix no decorations on startup on OS X
cmake: windows_copy_files
This should fix the GL loading errors that occur in some drivers due to
the use of deprecated functions by GLEW. Side benefits are more accurate
auto-completion (deprecated function and symbols don't exist) and faster
pointer loading (less entrypoints to load). In addition it removes an
external library depency, simplifying the build system a bit and
eliminating one set of binary libraries for Windows.
