Calling SDL_GL_GetAttribute when context construction has failed leads
to erroneous behavior. This call should only be made when a context has
been constructed correctly.
ContextAttribute.DOUBLEBUFFER is a boolean in SDL (false->single
buffering, true->double buffering). We need to adjust the number of
buffers accordingly (single buffering->1 buffer, double buffering->2
buffers).
SDL_GL_MakeCurrent(IntPtr.Zero, IntPtr.Zero) is the correct way to
remove the current OpenGL context from the calling thread. Fixes
threaded rendering on Windows.
This is a new platform that can be used then sdl2 is installed on the target system. SDL2 is commercially supported by Valve and provides better support for MacOS compared to our current implementation (Cocoa vs Carbon). It will also help us introduce faster support for new platforms. Existing platforms remain as a fallback and will be automatically used if sdl2 is not installed.
Please note that this is still a work in progress. The new mouse and keyboard API is not supported yet. Due to limitations of sdl2, multiple mice/keyboards are also not supported.
