* when using a shared gpu/cpu context on os x, CL_Rx/CL_RGx/CL_RGBx+CL_FLOAT image formats are falsely advertised as being supported -> set these to CL_R/CL_RG/CL_RGB manually
 * fixed order when creating the "opencl image format" -> "oclraster image format" mapping
 * added the possibility to specify an opencl image format when creating an oclraster image (the specified format will still be checked and will also fall back to buffer based backing when it's not supported); this is necessary, because opencl differentiates between CL_UNORM_*/CL_SNORM_* and CL_UNSIGNED_*/CL_SIGNED_* data types, but oclraster doesn't (as it should be, since the storage data type is the same in both cases and it should be possible to sample these images in normalized _and_ non-normalized mode); also note that oclraster will default to a CL_UNORM_*/CL_SNORM_* format (if supported) when no opencl image format is specified