[example] Simplifies the hello-world CL code to make it clearer
[example] Minor updates in hello-world examples
Adds new properties 'gl-context' and 'gl-display' to GoclContext
Adds gl_context and gl_display (if not null) to properties of GoclContext
Adds new constructor gpu_new_sync() to GoclContext
Adds create_cl_mem() virtual method to GoclBuffer class

This method will be used by GoclBuffer and derived classes to create the
actual cl_mem OpenCL object.
Adds new enum GoclImageType to common Gocl declarations header
Adds new class GoclImage, representing image-type buffers
Adds gocl_image_new() forward declaration to gocl-context headers
Adds new property 'host-ptr' to GoclBuffer
Includes a 'host_ptr' arg to create_cl_mem() virtual method of GoclBuffer
Adds CL_MEM_READ_ONLY to GoclBufferFlags enum
Adds new method set_argument_float() to GoclKernel
Adds new method list_to_array() to GoclBuffer

This is a rather low level method and should not normally be called by apps.
Adds new method acquire_gl_objects_sync() to GoclDevice
Adds new method release_gl_objects() to GoclDevice
[example] Minor update in hello-world-sync example
[build] Adds check for Cogl > 1.14 to configure, as an optional dependency
Adds new virtual method read_all() to GoclBuffer class
Adds implementation of read_all() virtual method in GoclBuffer