Reduce decoding time of embedded gltf + Fix base64 encode bug

Reduce decoding time of embedded gltf + Fix base64 encode bug

Let we don't copy the full string during decode base64.
+
Let we print '=' tags end of encoded base64 result, so
we can decode the encode result at common external library.

Change-Id: I2c671512ffcd986e30a4e773dacdd6e62dca2f31
Signed-off-by: Eunki, Hong <eunkiki.hong@samsung.com>