Use strrchr() when finding the '.' that begins the file extension.

Sometimes we encounter file paths like ../foo/bar.obj; searching from the end of
the string would yield the correct result.

samples/SimpleOpenGL/Sample_SimpleOpenGL.c

@@ -376,7 +376,7 @@ int main(int argc, char **argv)
 
 	// Check and validate the specified model file extension.
 	model_file = argv[1];
-	const char* extension = strchr(model_file, '.');
+	const char* extension = strrchr(model_file, '.');
 	if (!extension) {
 		print_error("Please provide a file with a valid extension.");
 		return EXIT_FAILURE;