Use a shared copy of placeholder textures, tweak placeholder appearance

This reduces memory usage a bit in case multiple placeholders were
requested, e.g. when using multiple NoiseTextures with no noise property
defined.

The placeholder texture's appearance was also changed from a plain magenta
color to a checkerboard alternating between magenta and black pixels.
This makes it easier to spot when the placeholder texture ends up
being used in a complex scene (usually by accident).
The texture's dimensions remain identical to keep the physical size
identical in 2D.

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