Fix normal vector error when we use skin

Fix normal vector error when we use skin

Since dali's world matrix use left-hand-coordinate,
the constrainted world matrix value in uBone need to be multiply negative Y.

To apply this information, let we use uYDirection value if we use skin.

Also, we need to use same precision if we use same uniform both vertex and
fragment shaders. To specify the precision, make uYDirection as mediump.

Change-Id: Ibf6ca7ee0cc88fba1214b4a68a0eea7a4805f7e5
Signed-off-by: Eunki, Hong <eunkiki.hong@samsung.com>