Eliminate "-Wsign-compare" warnings on Linux ...

With these being newly enabled for the loader, layers, and demos (and it turns
out, for the icd/common directory), it is appropriate to fix them.
diff --git a/demos/cube.c b/demos/cube.c
index 00951b5..42a9570 100644
--- a/demos/cube.c
+++ b/demos/cube.c
@@ -561,8 +561,8 @@
 {
   //header for testing if it is a png
   png_byte header[8];
-  int i, is_png, bit_depth, color_type,rowbytes;
-  png_uint_32 twidth, theight;
+  int is_png, bit_depth, color_type,rowbytes;
+  png_uint_32 i, twidth, theight;
   png_structp  png_ptr;
   png_infop info_ptr, end_info;
   png_byte *image_data;