Fix narrowing conversion warnings in device

This fixes a couple of narrowing conversion warnings in the
device partitioning methods which were seen when compiling
VexCL with Boost.Compute in C++11 mode.
Skip enqueue_write_buffer_rect() test on AMD GPUs

This skips the enqueue_write_buffer_rect() test on AMD GPUs which
don't correctly implement the clEnqueueWriteBuffer() function.
Check for partitioning support in device test

This adds checks to the device test-suite to ensure that the
current device supports the partitioning types before attempting
to use the corresponding device::partition_*() methods.