Use Wgl.Import.GetCurrentContext DllImport directly

Wgl is implemented using the regular DllImport + delegate approach, in
order to support extensions. The delegates have not been armed at this
point, so we need to use the DllImport directly.

The whole wgl API should be revisited (we are using only a tiny
portion.)
Add debug message when context creation fails
Use OpenGL window flag and switch to "fake" fullscreen mode

The OpenGL flag is required when using SDL2 on Windows. Fake fullscreen
works much better on modern monitors and systems with multiple monitors.
Increased the text surface size

This is necessary for high-dpi monitors which render text at double
resolution.
Implemented WindowState, WindowBorder and Icon

SDL2 suffers many of the issues we encountered when implementing
OpenTK.Platform.Windows. Workarounds are now in place to make
WindowState changes behave reasonably.