Lazily allocate RIDs for PlaceholderTextures to avoid allocating GPU resources unless used

[clayjohn]

Fixes: #77762

This may also fix unreported issues with PlaceholderTexture3D and the PlaceholderTextureLayered types.

The root of the issue here is that Godot instantiates resources to generate documentation in the editor. So the PlaceholderCubemapArray was being allocated on the GPU regardless of whether it was actually being used.

The get_rid() function is needed so that the texture types can actually be used. So I added it for PlaceholderTexture3D and PlaceholderTextureLayered. All other texture types lazily allocate RID like this so we don't allocate a texture on the GPU without actually using the texture.

[BastiaanOlij]

We indeed need to take far more care that in Godot objects, we shouldn't be doing any work in the constructor other than setting some default values.

[dsnopek]

Looks good to me! We're also using placeholders in the dedicated server export, and it's probably also beneficial in that context to not bother creating the RIDs (although, I guess it'd be using the dummy renderer anyway).

[YuriSizov]

Thanks!

[YuriSizov]

This can't be cherry-picked to 4.1 because in 4.2 we've split textures into dedicated files, but in 4.1 they are still in texture.h/cpp. A dedicated port would be required, if desired.