Depth Image Enums

[bashbaug]

See:

https://github.com/KhronosGroup/OpenCL-Docs/issues/284 https://github.com/KhronosGroup/OpenCL-Docs/pull/458

It looks like the enums for depth images in the headers have a similar problem:

#ifdef CL_VERSION_1_2
#define CL_DEPTH                                    0x10BD
#define CL_DEPTH_STENCIL                            0x10BE
#endif

We should:

• Move some of these enums out of OpenCL 1.2 and into OpenCL 2.0 in cl.h, since this is when they became a core feature.
• Move / duplicate a few other enums into cl_ext.h, since they are extension enums and are not part of a core API.