Support OpenCL image kernel parameters

[eyalroz]

We currently lack support for kernel parameters other than scalars and plain buffers.

Let us add support for the OpenCL memory objects image2d_t and image3d_t.

This would mean:

• distinguishing them from (other) buffers in the parameter description struct
• (hopefully not loading them differently)
• (possibly massaging their memory layout; but maybe not)
• Setting up the appropriate meta-data for them
• ... and reading relevant image meta-data for them, from the command-line
• Modifying the argument setting logic