Tilemap optimizations

[melody-rs]

Connections
#98
#102

Description
This PR overhauls how the tilemap and its components are rendered and used.

• The placeholder texture is now shared
• Vertex buffers for everything except sprites have been removed
• Instance buffers have been significantly shrunk or outright removed
• Only resources required for rendering are stored in an Arc, so all uses of AtomicCell have been removed
• Push constants code has been removed
• Uncapped layer size (layer opacity is now a dynamic uniform buffer)

Testing
Open maps, move events around, hover over events to display them

Checklist

• Run cargo fmt.
• Run cargo clippy. If applicable, add:
  • --target wasm32-unknown-unknown -Z build-std=std,panic_abort
• Run cargo build --release
• If applicable, run trunk build --release

[white-axe]

I get this panic message when using the web build with WebGL in both Chromium and Firefox:

The application panicked (crashed).
Message:  wgpu error: Validation Error

Caused by:
    In Device::create_render_pipeline
      note: label = `Tilemap Sprite Render Pipeline`
    In the provided shader, the type given for group 0 binding 2 has a size of 24. As the device does not support `DownlevelFlags::BUFFER_BINDINGS_NOT_16_BYTE_ALIGNED`, the type must have a size that is a multiple of 16 bytes.

Location: /home/h/.cargo/registry/src/index.crates.io-6f17d22bba15001f/wgpu-0.19.1/src/backend/wgpu_core.rs:3009

Luminol version: d6d5041e

Backtrace omitted. Run with RUST_BACKTRACE=1 environment variable to display it.
Run with RUST_BACKTRACE=full to include source snippets.

[melody-rs]

I get this panic message when using the web build with WebGL in both Chromium and Firefox:

The application panicked (crashed).
Message:  wgpu error: Validation Error

Caused by:
    In Device::create_render_pipeline
      note: label = `Tilemap Sprite Render Pipeline`
    In the provided shader, the type given for group 0 binding 2 has a size of 24. As the device does not support `DownlevelFlags::BUFFER_BINDINGS_NOT_16_BYTE_ALIGNED`, the type must have a size that is a multiple of 16 bytes.

Location: /home/h/.cargo/registry/src/index.crates.io-6f17d22bba15001f/wgpu-0.19.1/src/backend/wgpu_core.rs:3009

Luminol version: d6d5041e

Backtrace omitted. Run with RUST_BACKTRACE=1 environment variable to display it.
Run with RUST_BACKTRACE=full to include source snippets.

Weird, I don't get this panic. Should be an easy fix though!