Prevent out-of-bounds accesses in shaders

[leod]

Address this FIXME: https://github.com/leod/posh/blob/fbc3c3eb79f06939156a87a8d5640772ba318b71/src/sl/array.rs#L83

This is important in order to achieve soundness. Draw calls are not unsafe in posh, so shaders must avoid undefined behavior.