Fix the alignment issue of char3 data in WebCLBuffer.

For 3-component vector data types, the size of the data type is 4 * sizeof(component).
This means that a 3-component vector data type will be aligned to a 4 *
sizeof(component) boundary.
Merge pull request #87 from junmin-zhu/char3

Fix the alignment issue of char3 data in WebCLBuffer.