The Tight encoder uses the pixel buffer as a scratch pad, which doesn't

The Tight encoder uses the pixel buffer as a scratch pad, which doesn't
work so well with the new optimisation to feed it the raw frame buffer.
Reorganise and clean up the code to handle this, and make the raw frame
buffer pointer const so that we might avoid such bugs in the future.

git-svn-id: svn://svn.code.sf.net/p/tigervnc/code/trunk@4841 3789f03b-4d11-0410-bbf8-ca57d06f2519