fixes rendera_CXXFLAGS, adds AM_CXXFLAGS, source code tweaks to fix Werrors

edits ```Makefile.am``` to add ```$(AM_CXXFLAGS)``` to ```rendera_CXXFLAGS```

edits the following to quash the new errors coming from ```-Werror```:
* ```Blend.H```
* ```File.cxx```
* ```Gui.cxx```
* ```Paint.H```
* ```Palette.cxx```
* ```Project.H```
* ```Text.cxx```
* ```Tool.H```
* ```Undo.cxx```
* ```View.cxx```