Add new Preference, Maximum Number of Credits

Grant users the ability to change the maximum amount of credits available rather than the constant cap at 20.

(cherry picked from commit 2ec2355d7a7d9fec9410cf6b7d2b5c31a4406074)
(cherry picked from commit 48245ab51600ede16096e70e27c94345918eaed7)

src/ScreenSystemLayer.cpp

@@ -116,7 +116,7 @@ namespace
 					sCredits += ssprintf("  %d", iCredits);
 				if( PREFSMAN->m_iCoinsPerCredit > 1 )
 					sCredits += ssprintf("  %d/%d", iCoins, PREFSMAN->m_iCoinsPerCredit.Get() );
-				if( iCredits >= MAX_NUM_CREDITS )
+				if( iCredits >= PREFSMAN->m_iMaxNumCredits )
 					sCredits += "  " + CREDITS_MAX.GetValue();
 				return sCredits;
 			}