Add Wrapper field to gl_renderbuffer. Used for wrapping one renderbuffer

Add Wrapper field to gl_renderbuffer.  Used for wrapping one renderbuffer
with another, such as wrapping a hardware/win-sys RGB renderbuffer with a
software-based alpha buffer.
Previous alpha buffer wrapping was conflicting with the X driver's
xmesa_renderbuffer structure containment/inheritance.  That lead to
memory corruption.