Added a pause after the OS message loop

In Win32::MessageLoop() there is now a Sleep(100); before the return to
introduce a small break for the OS to do other things.
OpenGLSystem new renders element arrays

OpenGLSystem is now rendering a quad using an element array.
GLSprite has an offset now.

GLSprite has an offset parameter to draw it offset from the entity
position.

Alos the element buffer index is now stored in GLSprite.