Improve oil platform graphics
We've now got oil_rig graphics for all tilesets (feature #657386).
However, it doesn't look great, at least in the civ2civ3 ruleset. As noted in #694717, it's based on the oil_mine graphic that appears against pale Glacier and Desert, whereas oil_rig typically appears against dark Deep Ocean. So it's black against dark blue, with the most visible feature being occasional white speckles. It could really do with its contrast tweaking, at least.
Also, I wrote this about Amplio2 a while back:
This isn't entirely satisfactory; the sprite is a bit tall for Amplio2, which is particularly noticeable when you have coast directly below a tile with oil rig (rare, since in civ2civ3 that's only allowed on Deep Ocean).