Fix builds on older versions of SDL2

Source/palette.cpp

@@ -7,10 +7,6 @@
 #include "../SourceX/display.h"
 #include "../3rdParty/Storm/Source/storm.h"
 
-#ifdef USE_SDL1
-#include "sdl2_to_1_2_backports.h"
-#endif
-
 DEVILUTION_BEGIN_NAMESPACE
 
 SDL_Color logical_palette[256];

SourceS/sdl2_backports.h

@@ -3,6 +3,10 @@
 
 #include <SDL.h>
 
+#ifndef SDL_MAX_UINT32
+#define SDL_MAX_UINT32 ((Uint32)0xFFFFFFFFu)
+#endif
+
 #if !SDL_VERSION_ATLEAST(2, 0, 4)
 inline SDL_bool SDL_PointInRect(const SDL_Point *p, const SDL_Rect *r)
 {