pixman-gradient-walker: Make left_x and right_x 64 bit variables

pixman-gradient-walker: Make left_x and right_x 64 bit variables

The variables left_x, and right_x in gradient_walker_reset() are
computed from pos, which is a 64 bit quantity, so to avoid overflows,
these variables must be 64 bit as well.

Similarly, the left_x and right_x that are stored in
pixman_gradient_walker_t need to be 64 bit as well; otherwise,
pixman_gradient_walker_pixel() will call reset too often.

This fixes the radial-invalid test, which was generating 'invalid'
floating point exceptions when the overflows caused color values to be
outside of [0, 255].