Add string::c_str() to several functions.

There were a number of places where a std::string was being used in a function calls that expected a c string.  These instances were trivally fixed by adding a call to string::c_str().

Note, I'm not sure if this is a GCC specific issue, however even if it is a case of MSVC simply letting these usages slide it can't hurt to be explicit.

src/app/app.cpp

@@ -143,7 +143,7 @@ App::App(int argc, const char* argv[])
 
     base::UniquePtr<Palette> pal(Palette::load(palFile.c_str()));
     if (pal.get() == NULL)
-      throw base::Exception("Error loading default palette from: %s", palFile);
+	  throw base::Exception("Error loading default palette from: %s", palFile.c_str());
 
     set_default_palette(pal.get());
   }

src/app/commands/cmd_save_file.cpp

@@ -95,7 +95,7 @@ static void save_document_in_background(Document* document, bool mark_as_saved)
 
     StatusBar::instance()
       ->setStatusText(2000, "File %s, saved.",
-                      base::get_file_name(document->getFilename()));
+                      base::get_file_name(document->getFilename()).c_str());
   }
 }
 

src/app/file/file.cpp

@@ -246,13 +246,14 @@ FileOp* fop_to_save_document(Document* document)
   // Get the extension of the filename (in lower case)
   base::string extension = base::string_to_lower(base::get_file_extension(fop->document->getFilename()));
 
-  PRINTF("Saving document \"%s\" (%s)\n", fop->document->getFilename(), extension.c_str());
+  PRINTF("Saving document \"%s\" (%s)\n", 
+		 fop->document->getFilename().c_str(), extension.c_str());
 
   /* get the format through the extension of the filename */
   fop->format = get_fileformat(extension.c_str());
   if (!fop->format ||
       !fop->format->support(FILE_SUPPORT_SAVE)) {
-    fop_error(fop, "ASEPRITE can't save \"%s\" files\n", extension);
+	fop_error(fop, "ASEPRITE can't save \"%s\" files\n", extension.c_str());
     return fop;
   }
 

src/app/file/gif_format.cpp

@@ -325,7 +325,7 @@ bool GifFormat::onPostLoad(FileOp* fop)
                       "<<ASEPRITE cannot handle this kind of GIF correctly in Indexed format."
                       "<<What would you like to do?"
                       "||Convert to &RGBA||Keep &Indexed||&Cancel",
-                      fop->document->getFilename());
+                      fop->document->getFilename().c_str());
 
     if (result == 1)
       pixelFormat = IMAGE_RGB;

src/base/file_handle.cpp

@@ -28,7 +28,7 @@ FILE* open_file_raw(const string& filename, const string& mode)
   return _wfopen(from_utf8(filename).c_str(),
                  from_utf8(mode).c_str());
 #else
-  return fopen(filename.c_str(), mode);
+  return fopen(filename.c_str(), mode.c_str());
 #endif
 }