Simplify ctx->_NumColorDrawBuffers, _ColorDrawBuffers and fix bug 13835.

Simplify ctx->_NumColorDrawBuffers, _ColorDrawBuffers and fix bug 13835.

These fields are no longer indexed by shader output.  Now, we just have
a simple array of renderbuffer pointers.

If the shader writes to gl_FragData[i], send those colors to the N
_ColorDrawBuffers.  Otherwise, replicate the single gl_FragColor (or
the fixed-function color) to the N _ColorDrawBuffers.

A few more changes and simplifications can follow from this...