Improvements

Improved error handling in the DB module.

Calling init_gl() now declares the Titania::GL constant, which
references the current GL implementation, allowing constant lookup in
the manner of GL::GL_xxxx.

Added new gl_do(){|gl_impl| ... } method.  This will safely handle
locking and unlocking the shared GL context for calls to OpenGL that
must be performed independent of the renderer.  Errors are returned, and
the context is released regardless.