Modify default shader according to the gltf pbr spec

 - Add uColorFactor.
   In glTF, color factor can be used as a color when there isn't color texture,
   but it is also used to be multiplied with base color that is sampled from color texture.
   In previous implementation, when there isn't color texture, color factor is converted
   as single pixel texture.
   If the texture is from color factor, we shouldn't multiply color factor again.
   But, we couldn't notify whether the texture is from image file or color factor.

 - Add vertex color

 - Add uMetallicFactor, uRoughnessFactor

 - Use #ifdef instead of single value texture.

 - Use pre-computed brdf texture.

 - Modify alpha mode

Change-Id: I048b793a4481b65da5ce07d0d3e263ae74b2ab59
Signed-off-by: seungho <sbsh.baek@samsung.com>