This is not currently supported. It may be taken out, since C is unlikely to let us do it easily.
If you need to create a special type, here's how to do it. It's not recommended that you do it, for the following reasons:
unsigned char mark1 : 1; /* user defined marker 1 */
unsigned char mark2 : 1; /* user defined marker 1 */
unsigned char mark3 : 1; /* user defined marker 1 */
unsigned char isblock : 1; /* is part of a block? */
unsigned char ischar : 1; /* is part of a character? */
unsigned char private1: 1; /* internal field. */
unsigned char private2: 1; /* internal field. */
/* your data goes here */
typedef struct mypixel MyPixel;
More: struct size, etc.