SamplerState is now only the members that are overridden by a sampler
object, this makes it easy to update those specific members.

Opted for getters and setters for each member in Texture and Sampler
because it will be required to enable dirty bits for these states.

Added maxAnisotropy to the SamplerState instead of texture state.  The
sampler objects extension mentions it should be there.

BUG=angleproject:1162

Change-Id: I5aa6d702bd5915ee9df1976afef3c8c1f69d27c8
Reviewed-on: https://chromium-review.googlesource.com/300490
Tryjob-Request: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>