[Gungriffon] Improved mesh doesn't take sprite color calculation ratio in account

[fafling]

As reported by @gridmortis in #1281, turning on improved mesh displays bands of lighter colors on the ground. This is present in public v2.5.0 and in the last WIP.

With improved mesh :

Without improved mesh :

The game uses polygons that it draws over the RBG0 ground with various color calculation ratios to create a light level gradient. It uses mesh every other polygons to create intermediate levels in this gradient.

The problem is that improved mesh seems to ignore the color calculation ratio that is set on those mesh areas, and always makes a half transparency blending which makes the mesh polygons much more visible than they should be.

The actual sprite color calculations ratios are all different of 16:16, and almost all much lower :

Tested with the original US CD.

[fafling]

The correct behavior would be that improved mesh halves the color calculation ratio that is set on the sprite pixel. 16:16 should only be used when color calculation is disabled on that pixel.