[Flutter GPU] Storage buffer bindings #188479

Part of #188474.

Compute shaders need storage buffers bound from Dart. After #188477, Shader holds the reflected storage buffers, but nothing exposes them.

Changes

• A storage-buffer slot on Shader, alongside the existing uniform and texture slots, with the same cached index lookup.
• Storage buffers in binding sets ([Flutter GPU] Reusable binding sets #190400).
• Bind-time validation, since out-of-bounds storage access is undefined on the GPU:
  • the view is at least the block's size_in_bytes;
  • for a block ending in a runtime-sized array, the remainder is a whole number of elements;
  • the offset meets the device's minimum storage-buffer offset alignment, which gpuContext exposes.
• HostBuffer storage emplacements use the storage alignment instead of the uniform alignment.

The bind method itself lives on ComputePass (#188480). Storage buffers in vertex and fragment shaders wait for the OpenGL ES tiering in Wave 3 of #188474.

Testing

Slot lookup for storage buffers, including an optimized-out buffer, and rejection of a short view, a partial trailing element and a misaligned offset.

Dependencies

#188477.