gradient walker: Correct types and fix formatting

gradient walker: Correct types and fix formatting

The type of pos in gradient_walker_reset() and gradient_walker_pixel()
is pixman_fixed_48_16_t and not pixman_fixed_32_32. The types of the
positions in the walker struct are pixman_fixed_t and not int32_t, and
need_reset is a boolean, not an integer. The spread field should be
called repeat and have the type pixman_repeat_t.

Also fix some formatting issues, make gradient_walker_reset() static,
and delete the pointless PIXMAN_GRADIENT_WALKER_NEED_RESET() macro.