[MP] fix water jumping discrepancy

[Razish]

reverts a change introduced in Razish/jasdk_modbase@09edabf but provide an alternative fix that better matches SP code/behaviour with as few changes as possible this lets you jump out of the water pools in taspir2 like you can in SP introduces g_fixWaterJump (default: 0) / LEGACYFIX_WATERJUMP

To be clear, OpenJK modules currently let you jump out of pools because we "fixed" a contents check. They shouldn't. But rather than remove that ability outright, I'm putting it behind a legacy fix option that better matches SP code.

discovered by @CansecoDev

SP: https://github.com/JACoders/OpenJK/assets/844370/5ebdd10e-7ca5-4ab9-bf3e-c581b5934c25

MP original behaviour: https://github.com/JACoders/OpenJK/assets/844370/2423b20b-52ae-4ab3-ba95-ad38ccff5e6d