mesa: Don't lose track of the shader image layer originally specified by the user.

mesa: Don't lose track of the shader image layer originally specified by the user.

The spec requires that all layers of the image starting from the 0-th
are bound to the image unit regardless of the Layer parameter when
Layered is true, so I was setting gl_image_unit::Layer to zero in that
case for the convenience of the driver back-end.  However the
ES31-CTS.shader_image_load_store.basic-api-bind conformance test
checks that the layer value returned by glGetInteger is the same that
was originally specified, regardless of the value of layered.  Rename
Layer to _Layer as is usual for other derived state and keep track of
the original layer value as gl_image_unit::Layer.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>