Make the NavigationAgent3D icon more readable

[MewPurPur]

It looked unreadable because of the 3D-ness. The 3D-ness was actually added because the icons used to have no other discerning quality, but now there's the color too.

Red-blue colorblindness is extremely rare, while small screens are common

Red and blue are opposite on the visible color spectrum. Even if both chromosomes fail to code for blue sensors, the resulting tritanopia can still discern red from blue.

The other option for red-blue colorblindness is if both the red and the green sensors are absent, which is extremely rare. This would mean total color blindness, not just red-blue color blindness. This type of colorblindness would struggle to discern our 2D's red and 3D's blue.

To be honest, I don't think confusability of 2D and 3D nodes matters that much. As a casual Godot user, you can hide 2D nodes if you're making a 3D game, but you can't fix an icon that's hard to read because of its 3D-ness.

The author of the icons suggested I change NavigationObstacle3D too for consistency, so I did.

[akien-mga]

Thanks!