Add DebuggerHidden attribute to CheckGLError
The DebuggerHidden attribute makes the debugger break at the caller of CheckGLError if an exception is thrown, not inside CheckGLError.  This is much more helpful to devs as they do not have to manually navigate up the callstack.
Fix using statement.
Merge pull request #1730 from slygamer/debuggerhidden

Add DebuggerHidden attribute to CheckGLError
Merge pull request #76 from mono/develop

Merge back from develop