Shrinking a sprite causes stamp lines to disappear (impacts "Vectors Demo" microworld)

[rigdern]

Repro steps

I reproed this with Boxer 3.4.18 2024-05-26 on macOS

1. Download scaling sprites.box.zip, unzip it, and open scaling sprites.box.
2. Click in the box named scaling sprites to view it.

Unexpected result: As the demo shows, the circles rendered by stamp-hollow-circle are partially clipped when sprite-size is 0.75 and are entirely invisible when sprite-size is 0.5:

Impact

This impacts the "Vectors Demo" microworld. The Earth is missing its border.

Repro steps:

1. Open the "Vectors Demo" microword file.
2. In Boxer, click the microwold's box to navigate into it.
3. Click the "Apollo Missions - Flight to the Moon" box to navigate into it.
4. Click "earth-orbit" in the "menu" box to view that scenario.

Unexpected result: The Earth is missing its border (the sprite-size is 0.5) (to see any graphics at all, you might have to resize the graphics box due to issue #79):

Workaround. As a workaround to see the Earth with a border, change its sprite-size to 1 by running: tell earth set-sprite-size 1:

[sgithens]

I think this may be related to an issue with a GL Shader I wrote for circles/ellipses, and likely also affects rotating/app-zooming. Looking in to it...