Make TextureHash as buffer

Previous hashTarget use std::string, and Dali::CalculateHash function use
.c_str() internally. This logic will ignore zero values.
For example, when desired_width is 256, TextureHash result will ignore
whole informations about desired_size, fittingMode, maskId, etc.

This patch Fix that problem so we can seperate un-matched textures
and also reduce hash collision.

Change-Id: I95526f7144991e630ac0d19abaefc50175b4317a
Signed-off-by: Eunki, Hong <eunkiki.hong@samsung.com>