SDL Mouse Button clean up

Make 'b' variable an int so that key values more than 255 will work (not currently an issue in ioq3 afaik).
Use SDL_BUTTON_* for easier transition to SDL2.
(SDL2 doesn't have SDL_BUTTON_WHEELUP or SDL_BUTTON_WHEELDOWN, X1/X2 are different values.)

code/sdl/sdl_input.c

@@ -912,17 +912,17 @@ static void IN_ProcessEvents( void )
 			case SDL_MOUSEBUTTONDOWN:
 			case SDL_MOUSEBUTTONUP:
 				{
-					unsigned char b;
+					int b;
 					switch( e.button.button )
 					{
-						case 1:   b = K_MOUSE1;     break;
-						case 2:   b = K_MOUSE3;     break;
-						case 3:   b = K_MOUSE2;     break;
-						case 4:   b = K_MWHEELUP;   break;
-						case 5:   b = K_MWHEELDOWN; break;
-						case 6:   b = K_MOUSE4;     break;
-						case 7:   b = K_MOUSE5;     break;
-						default:  b = K_AUX1 + ( e.button.button - 8 ) % 16; break;
+						case SDL_BUTTON_LEFT:		b = K_MOUSE1;     break;
+						case SDL_BUTTON_MIDDLE:		b = K_MOUSE3;     break;
+						case SDL_BUTTON_RIGHT:		b = K_MOUSE2;     break;
+						case SDL_BUTTON_WHEELUP:	b = K_MWHEELUP;   break;
+						case SDL_BUTTON_WHEELDOWN:	b = K_MWHEELDOWN; break;
+						case SDL_BUTTON_X1:			b = K_MOUSE4;     break;
+						case SDL_BUTTON_X2:			b = K_MOUSE5;     break;
+						default:  b = K_AUX1 + ( e.button.button - SDL_BUTTON_X2 + 1 ) % 16; break;
 					}
 					Com_QueueEvent( 0, SE_KEY, b,
 						( e.type == SDL_MOUSEBUTTONDOWN ? qtrue : qfalse ), 0, NULL );