ogsf: fix possible overflow errors in gvld module

We were doing `(255 << 24)` which causes integer overflow
and positive number gets converted to negative number. We
were then assigning this to an unsigned integer in multiple
places, which does conversion in a different way.

For example: If we do unsigned int x = -20, `UINT_MAX + 1 - 20`
is assigned to x.

I do not think that's what is intended when we do
`ktrans = (255 << 24)`. Fix instances of that, by using an
unsigned int literal over int literal.

This issue was found using cppcheck tool.

Signed-off-by: Mohan Yelugoti <[email protected]>

lib/ogsf/gvld.c

@@ -188,7 +188,7 @@ int gvld_isosurf(geovol *gvl)
 
         /* transparency */
         check_transp[i] = 0;
-        ktrans[i] = (255 << 24);
+        ktrans[i] = (255U << 24);
         if (CONST_ATT == isosurf->att[ATT_TRANSP].att_src &&
             isosurf->att[ATT_TRANSP].constant != 0.0) {
             ktrans[i] = (255 - (int)isosurf->att[ATT_TRANSP].constant) << 24;