PCG jump incorrectly emulates uint128 shift

[runewalsh]

In random/src/pcg64/pcg64.c, uint128 version (PCG_EMULATED_128BIT_MATH disabled) performs:

delta /= 2;

uint64+64 version (PCG_EMULATED_128BIT_MATH enabled) emulates it with manual delta >>= 1ing:

delta.low >>= 1;
delta.low += delta.high & 1;
delta.high >>= 1;

This is obviously incorrect and will screw up jumps by 2⁶⁴ or more (those with delta.high != 0). Must be rather:

delta.low = (delta.low >> 1) | (delta.high << 63);
delta.high >>= 1;

[bashtage]

Definitely a bug.