glsl/apps: print usage info if insufficient args

src/glsl/apps/process.c

@@ -47,6 +47,7 @@ main(int argc,
    unsigned int i;
 
    if (argc != 3) {
+      printf("Usage: process infile outfile\n");
       return 1;
    }
 

src/glsl/apps/purify.c

@@ -45,6 +45,7 @@ main(int argc,
    FILE *out;
 
    if (argc != 3) {
+      printf("Usage: purify infile outfile\n");
       return 1;
    }
 

src/glsl/apps/tokenise.c

@@ -46,6 +46,7 @@ main(int argc,
    unsigned int i;
 
    if (argc != 3) {
+      printf("Usage: tokenize infile outfile\n");
       return 1;
    }
 

src/glsl/apps/version.c

@@ -45,6 +45,7 @@ main(int argc,
    FILE *out;
 
    if (argc != 3) {
+      printf("Usage: version infile outfile\n");
       return 1;
    }