The read buffer state is part of the framebuffer object, so determining
the index is moved to the Framebuffer class. Also make sure GL_NONE is
handled without causing invalid access by using GL_INVALID_INDEX.

Change-Id: I26476024cd8dd820b0e3f53d9c3dd5a7d46c082e
Reviewed-on: https://swiftshader-review.googlesource.com/14748Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>