The lod level should be selected according to the explicit gradients,
and the gradients are relative to the normalized texture coordinates,
so they need to be multiplied by the base level dimensions to get the
correct gradients to use in the lod level formula.

In the case of sampling integer cube maps, the derivatives of the
texture coordinates on the cube map face need to be calculated based
on the derivatives of the cube map direction vector components.

Also includes fix for sampling integer cube maps with explicit LOD.

BUG=angleproject:1092
TEST=dEQP-GLES3.functional.shaders.texture_functions.*grad*

Change-Id: Iadd358e713fa9695e755e98db8f368e8c512ac45
Reviewed-on: https://chromium-review.googlesource.com/337100Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>