shader: Work around indirect image access being not allowed with GLES

Resolve the indirect access by using a switch statement instead of the
indirect addressing.

Fixes: arb_arrays_of_arrays/execution/image_store/
    basic-imageStore-*-const-uniform-index

Signed-off-By: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
1 file changed