Fix r_glsl_dumpshader segfault

[hemebond]

Fixed #145 by changing dupe variable back from unsigned int to int.

[bones-was-here]

Good catch, in 05bc755 I should have changed only i to unsigned (needed to prevent a warning), didn't notice that dupe is counting backwards. Rereading it now, the rest of that commit looks OK.

I think mode should also be changed back to signed because its initial value is 0 so with it unsigned, mode - 1 gives 4294967295u, when this is cast back to signed for initialising dupe signed overflow occurs (though it gives the desired result of -1 on typical CPUs).
For language it doesn't matter if it's signed or unsigned.