Protect against recursion in Closing event

Misbehaving clients that shall not be named here may call
GameWindow.Close() inside the GameWindow.Closing event. This causes
recursion in SDL2, crashing the application.

This patch adds a guard to protect against recursion when calling
GameWindow.Close().
Added hack for MonoGame compatibility

MonoGame uses reflection to access internal APIs. We need to keep these
available until this is fixed downstream.
Improved diagnostics

This helps when tracking down GraphicsContext-related issues in SDL2.
GraphicsContext cannot be released by finalizer

On many/most platforms, GraphicsContexts can only be released by the
thread where they are current. This means that the user must call
GraphicsContext.Dispose() or risk a resource leak.

Since we cannot release contexts on the finalizer thread, we should keep
strong references, instead of weak references, until the user explicitly
calls Dispose().

This patch fixes issues with SDL2 crashing when running the MonoGame
WindowsGL test suite.
Use 16bit depth in GraphicsMode.Default

16bits appear to be better supported than 24bits on virtual machines.
Updated author information
Updated author information (2/2)
Marshal strings as UTF8 (affects #18)

Starting with OpenGL 4.2, strings passed to GL.ShaderSource are allowed
to contain multi-byte characters in comments (issue #18). This patch
modifies the marshaling code to use UTF8.GetBytes in order to marshal
strings, instead of Marshal.StringToHGlobalAnsi().
Clean up shader error detection
Corrected the entrypoint for wglChoosePixelFormat
Use opengl32 instead of gdi32 throughout

Since we are dynamically loading opengl32.dll, we are supposed to use
the wgl version of functions that exist in both opengl32 and gdi32 dlls.
Minor code cleanup

No need to wrap Wgl.DescribePixelFormat, just call it directly.
Load opengl32.dll before gdi32.dll

According to
http://stackoverflow.com/questions/199016/wglcreatecontext-in-c-sharp-failing-but-not-in-managed-c,
opengl32.dll must be loaded before gdi32.dll. Affect issue #19.
Use gdi32 implementations of functions

Several functions are defined in both gdi32 and opengl32. Using the
opengl32/wgl versions did not appear to help with issue #19. Let's use
the gdi32 version instead, as suggested here:
https://www.opengl.org/wiki/Platform_specifics:_Windows#The_WGL_functions
Minor code cleanup
Fixed GetModesARB implementation

The correct way to query number of available pixel formats is to use
Wgl.Arb.GetPixelFormatAttrib(NumberPixelFormatsArb), not
Wgl.Arb.ChoosePixelFormats. This fixes an issue where Intel drivers
would fail to report any pixel formats in GetModesARB, even when
WGL_ARB_pixel_format is supported.
Cleaned up temporary context construction

The temporary context is now retained until the actual context has been
constructed. If we don't do this, then WGL_ARB_create_context may fail
to work correctly on specific GPUs (e.g. Intel). This may affect issue
#19.
No point in using 16bpp color
Replace Wgl.GetCurrentDC with cached dc
 with
Implemented SDL2 Joystick and GameController events