filamat: Resolve string processing inefficiency (#2754)

Co-authored-by: Michael Lou <ylou@amazon.com>

libs/filamat/src/eiff/LineDictionary.cpp

@@ -55,7 +55,7 @@ void LineDictionary::addText(const std::string& line) noexcept {
             cur++;
             len++;
         }
-        std::string newLine(s, pos, len);
+        std::string newLine(s + pos, len);
         addLine(std::move(newLine));
         cur++;
     }

libs/filamat/src/eiff/MaterialTextChunk.cpp

@@ -46,7 +46,7 @@ void compressShader(const char *s, Flattener &f, const LineDictionary& dictionar
             len++;
         }
 
-        std::string newLine(s, pos, len);
+        std::string newLine(s + pos, len);
 
         size_t index = dictionary.getIndex(newLine);
         if (index > UINT16_MAX) {