loader.cpp: improved file extension checking, made Upper/LowerStr useful

Instead of forcibly taking the last 4 characters, it now finds the last extension separator (the period) and takes a substr of its location.

src/common/string_util.cpp

@@ -18,19 +18,15 @@
 #endif
 
 /// Make a string lowercase
-void LowerStr(char* str) {
-    for (int i = 0; str[i]; i++) {
-        str[i] = tolower(str[ i ]);
-    }
+std::string LowerStr(std::string str) {
+    std::transform(str.begin(), str.end(), str.begin(), ::tolower);
+    return str;
 }
 
 /// Make a string uppercase
-void UpperStr(char* str) {
-    for (int i=0; i < strlen(str); i++) {
-        if(str[i] >= 'a' && str[i] <= 'z') {
-            str[i] &= 0xDF;
-        }
-    }
+std::string UpperStr(std::string str) {
+    std::transform(str.begin(), str.end(), str.begin(), ::toupper);
+    return str;
 }
 
 // faster than sscanf