glsl: force creation of new scope for for-loop body

Fixes regression in progs/demos/convolution.c due to loop unrolling.
This also allows the following to be compiled correctly:
for (int i = 0; i < n; i++) {
   int i;
   ...
}
This fix is a bit of a hack, however.  The better fix would be to change
the slang_shader.syn grammar.  Will revisit that...
1 file changed