Fix empty line handling in line directive callback.

The current line number for the #line directive should be passed
in as parameter to the line directive callback. Without it, we
don't know how many empty lines we should output.

Test/baseResults/preprocessor.line.frag.out

@@ -1,5 +1,4 @@
 #version 310 es
-
 #line 1 2
 #pragma something
 void main(){ }