etnaviv: shared resource improvements

This improves handling of resource that are shared outside of the screen. Optimize the implicit flush path a bit more by only engaging those flushes for shared resources. When texturing from shared resources we need to make sure that internal shadows are updated with potentially new data at the first use after a context flush, which covers the sync requirements discussed at https://github.com/KhronosGroup/OpenGL-Registry/issues/566.