pixman-utils.c, pixman-private.h: Add floating point conversion routines

pixman-utils.c, pixman-private.h: Add floating point conversion routines

A new struct argb_t containing a floating point pixel is added to
pixman-private.h and conversion routines are added to pixman-utils.c
to convert normalized integers to and from that struct.

New functions:

  - pixman_expand_to_float()
    Expands a buffer of integer pixels to a buffer of argb_t pixels

  - pixman_contract_from_float()
    Converts a buffer of argb_t pixels to a buffer integer pixels

  - pixman_float_to_unorm()
    Converts a floating point number to an unsigned normalized integer

  - pixman_unorm_to_float()
    Converts an unsigned normalized integer to a floating point number