Fix slight wobble in temperature caused by float.

The transition variable 'adjustment_alpha' is a float. Redshift will wobble between 6499 and 6500K as the float runs out of precision. Using a double seems to solve the issue.
(Thanks to Sean Hildebrand)
colorramp: Fix slight flicker caused by improper scaling of gamma ramp values

Values generated by colorramp_fill() will be a maximum of i * 256 - 1, due  to the use of UINT16_MAX. Simplest fix ought to be using (UINT16_MAX+1), which is safe, as the maximum value (255 * 256) still fits in a uint16.
(Thanks to Sean Hildebrand)