fix ground primitives crossing IDL in 2D

[likangning93]

Fixes #6951

In Sandcastle

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[likangning93]

This is a branchless equivalent to:

if (planes2D_high.x > planes2D_high.w) {
    // If the primitive is split across the IDL
    if (position3DLow.y > 0.0) { // Comparison with position3DHigh may produce artifacts
        // If this vertex is on the east side of the IDL:
        // - existing "east" is on the wrong side of the world, far away
        // - set "east" as beyond the eastmost extent of the projection
        planes2D_high.w = EAST_MOST_X_HIGH - (WEST_MOST_X_HIGH - planes2D_high.w);
        planes2D_low.w = EAST_MOST_X_LOW - (WEST_MOST_X_LOW - planes2D_low.w);
    } else {
        // If this vertex is on the west side of the IDL:
        // - existing "west" is on the wrong side of the world, far away
        // - set "west" as beyond the westmost extent of the projection
        planes2D_high.x = WEST_MOST_X_HIGH - (EAST_MOST_X_HIGH - planes2D_high.x);
        planes2D_low.x = WEST_MOST_X_LOW - (EAST_MOST_X_LOW - planes2D_low.x);
    }
}

[likangning93]

Gist of what the problem was:

• when primitives are split over the IDL, the positions for "east" and "west" that get added to the batch table are actually flipped
• so the "planes" used to compute texture coordinates/do culling are really silly

Gist of the fix:

• we pass the furthest "west" and furthest "east" positions into the vertex shader as constants
• In the vertex shader, detect IDL-split based on the batch table planes
• In the vertex shader, determine which side of the IDL the vertex is on. It's guaranteed that all connected geometry will be on one side or the other because of the IDL offset in GeometryPipeline
• In the vertex shader, move the plane that's "wrong" for this side of the primitive using the constants passed in.

[ggetz]

Just a reminder that if we want this in for the next release, it should be merged today.

[bagnell]

👍

Don't forget about the issue where the volumes need to be closed 😃