Support Per-Axis Sampler Repeat Mode

[IsaacMarovitz]

Closes godotengine/godot-proposals#246, #44553, and #51625

Supporting this feature requires quite a few fundamental changes to parts of the core.

• Rather than building a fixed list of a small subset of possible sampler permutations in material_storage, samplers can now be created as needed by calling material_storage->sampler_rd_get(). They are stored in a HashMap with the RD::SamplerState as the key, ensuring that duplicate samplers are never made. This HashMap is never trimmed unless the whole list is delloc'd. It's, therefore, possible to build a huge number of sampler variants by requesting many different states, but I don't think this will be much of an issue in reality.
• samplers_inc.glsl is removed, as this fixed subset of samplers no longer exists. This means that every shader is expected to bind the required samplers as needed.
• DefaultIdentifierActions.custom_samplers has been renamed to just samplers and can be used to define a mapping to an existing sampler binding when needed
• DefaultIdentifierActions.default_filter and DefaultIdentifierActions.default_repeat have been removed
• GLTF & FBX document classes have been updated to use the correct sampler state, resolving an unsupported part of their respective specs.

TODO:

• Fix missing canvas samplers
• Fix up missing changes in Fog, Sky, and GI
• Convert shaders using append_uniforms
• Finalise hashing method
• Finalise shader changes
• Fix up mobile renderer
• Sort out anisotropy and LOD bias

[IsaacMarovitz]

Open to suggestions for better syntax here.