Rendering improvements related to liquids

[grorp]

Instead of using of the whole texture, only the first frame of animated tiles is used. This for { type = "vertical_frames" } tile animations only.

Changes this

into this

I tried to modify this Minetest client in order to learn some Rust and then made a pull request because why not.

[grorp]

I've added more more rendering improvements related to liquids to this PR:

• Flowing liquids now only use the first two textures from special_tiles, as they should.
• Faces of flowing liquids are now, like those of liquid sources, culled if their neighbor is a cube or the same node.
• Faces of both liquid sources and flowing liquids are now also culled if their neighbor is their "alternative form", i.e. their liquid_alternative_flowing or liquid_alternative_source.

Another comparison:

before:

after:

[grorp]

And another comparison:

before:

after:

[LizzyFleckenstein03]

Hi, thanks for your PR, I'm sorry I only noticed it now.

This will be merged soon-ish. I'm currently working with luatic on texture modifiers, and I want to use texture modifiers to extract the verticalframe from the texture. This way I can rely on the same caching mechanism for all sorts of modifications that are done to an image, so that will be done first.

Also note that I have previously determined HashMap lookups to be a major bottleneck in the create_mesh function (flamegraph revealed it), hence the weird [Option<Box<NodeDef>>; u16::MAX as usize + 1] construct. This means that the node_names_to_ids will be replaced by an array lookup as well.

You don't need to fix or update this PR, I will do it when I merge it. The best way people can currently help me is by researching and documenting minetest behavior, so this PR is helpful, thanks :)