Introduce enum class for texture type.

This represents the GL texture "target" but at the API-neutral level. It
will be needed here because proxy's that wrap imported texture's need to
know about sampling restrictions.

Change-Id: Ie811a6f6d04ba1b04faa6908422dca64e8e447c8
Reviewed-on: https://skia-review.googlesource.com/144304
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
19 files changed