WebCLKernelHandler: don't make a size argument for an image argument

This broke the accepting of image2d_t arguments to the kernel (and
actually trying to call the code while assuming there was no added
parameter).
kernel-runner: added support for image arguments
WebCLTransformer: add a check around WriteImage

This fixes issue #66.

Also declared write_imagef in kernel.cl for image2d_t.
Merge pull request #94 from eras/erkkise/#66

Fix #66: write_imagei/ui/f checked for valid range Failing test is problem of pocl.