Fragment output fix

Allow not specifying a fragment output location.

Change-Id: I76c6e6efdff3161586f5d5422e97731969b9396c
Reviewed-on: https://swiftshader-review.googlesource.com/17028
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
diff --git a/src/OpenGL/compiler/ParseHelper.cpp b/src/OpenGL/compiler/ParseHelper.cpp
index 4f6ffff..f2f3089 100644
--- a/src/OpenGL/compiler/ParseHelper.cpp
+++ b/src/OpenGL/compiler/ParseHelper.cpp
@@ -2826,7 +2826,7 @@
 {
 	TLayoutQualifier qualifier;
 
-	qualifier.location = -1;
+	qualifier.location = -1;  // -1 isn't a valid location, it means the value isn't set. Negative values are checked lower in this function.
 	qualifier.matrixPacking = EmpUnspecified;
 	qualifier.blockStorage = EbsUnspecified;