shader: Fix GLES image failures
This is v2 of this series: https://patchwork.freedesktop.org/series/47775/
I "chickened out" a bit, and only did the layout-binding approach on GLES, as it was very obvious how to do this. For GL 4.2 and up, there's more to it.
No dEQP regressions on my setup for GLES-on-GL nor GLES-on-GLES.
Edited by Erik Faye-Lund