(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

settings_gui.c

@@ -6,6 +6,7 @@
 #include "gfx.h"
 #include "command.h"
 #include "engine.h"
+#include "screenshot.h"
 
 static uint32 _difficulty_click_a;
 static uint32 _difficulty_click_b;