Buffer mod.: add OCL 1.1/1.2 function wrappers

-Added functions:
 * ccl_buffer_new_from_region()
 * ccl_buffer_enqueue_read_rect()
 * ccl_buffer_enqueue_write_rect()
 * ccl_buffer_enqueue_copy_rect()
 * ccl_buffer_enqueue_fill()
- These functions are not yet tested.
- Update OpenCL stub with the respective wrapped
OpenCL functions.