mesa: Expose function to calculate whether a shader image unit is valid.
A future commit will remove all texture object-dependent derived state from the image unit struct to make validation unnecessary on texture state changes. Instead of checking gl_image_unit::_Valid drivers will be required to call this function when needed to find out whether an image unit is in a valid state and whether access from the shader is allowed. Tested-by:Ye Tian <yex.tian@intel.com> CC: "11.0" <mesa-stable@lists.freedesktop.org> Reviewed-by:
Ian Romanick <ian.d.romanick@intel.com>