Encapsulate textures into their own Texture class

the main reason for doing this is so that we can have
access to informations about a texture (like its dimension)
close to where we generate and use shaders in ES 2.0.
Previously, there wasn't any way to get to a texture's size
from a RenderEngine implementation.

Bug: 8679321

Change-Id: I388b338a70d07e3e8177dde248710ea1e4c82dff