The base level is passed to shaders in an array included in the driver
uniform block. This is done on feature levels above 9_3, which treat
samplers as indices to sampler arrays in shaders.

A separate uniform block couldn't be used for the sampler metadata,
since that would bring the number of available uniform blocks down to
below minimum level defined by GLES 3.0.

BUG=angleproject:596
TEST=angle_end2end_tests

Change-Id: Ie040521402f9996d51a978aeeba9222e9dd761ce
Reviewed-on: https://chromium-review.googlesource.com/326290Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>