Clear a stale background flag before each left click in point segmentation

[mattdawkins]

• Every Point-mode edit layer listens for middle mousedown on the document, but only the layer under the cursor created the negative point and cleared its flag; the other camera's layer (or any layer when the middle click landed off-canvas) kept it, so the next left click there was a red background point
• The native handler now sets the flag from the current button, so a left click clears leftover state before GeoJS runs
• Spec covers a middle click on one Point layer followed by a left click and a shift click on another

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