Fl_Text_Display C code simple refactoring:

+ Replaced uninitialized C parameters archaisms, by on the fly C++ local var. decl.
  Not only it will be more optimized by modern c++ compilers,
  code is easier to read, maintain and debug and is shorter.
  Most local variables are now explicitly intialized when declared.
+ Replaced some non const vars by const equivalents when required.


git-svn-id: file:///fltk/svn/fltk/branches/branch-1.3@6825 ea41ed52-d2ee-0310-a9c1-e6b18d33e121