Implicit drawing operations (like window borders) are done by accessing

Implicit drawing operations (like window borders) are done by accessing
the screen pixmap directly. That means that hooking windows is insufficient.
The code now assumes that the drawable is a visible window, or the screen
pixmap (IOW, that all coordinates are screen relative). Had to restructure
some code that only worked on windows to get this working.

git-svn-id: svn://svn.code.sf.net/p/tigervnc/code/trunk@4941 3789f03b-4d11-0410-bbf8-ca57d06f2519