utils.[ch]: Add new format_name() function

utils.[ch]: Add new format_name() function

This function returns the name of the given format code, which is
useful for printing out debug information. The function is written as
a switch without a default value so that the compiler will warn if new
formats are added in the future. The fake formats used in the fast
path tables are also recognized.

The function is used in alpha_map.c, where it replaces an existing
format_name() function, and in blitters-test.c, affine-test.c, and
scaling-test.c.