(svn r631) Merge r440, r444, r485, r630 to trunk:

r440
Move screenshot function declarations to new file screenshot.h
Clean up screenshot.c a bit, mostly whitespace, alloca() -> malloc() and checking return values
r485
Remove unused field from struct ScreenshotFormat

strings.c

@@ -4,6 +4,7 @@
 #include "town.h"
 #include "vehicle.h"
 #include "news.h"
+#include "screenshot.h"
 
 #define USE_TABLE(x)  { assert(index < lengthof(x)); str = x[index]; break; }