Decouple compute GSplat renderer from work buffer via setDataSource

[mvaligursky]

Decouples GSplatComputeLocalRenderer from GSplatWorkBuffer by introducing a setDataSource API, allowing the compute renderer to work with any object providing { format, getTexture(name) }.

Changes:

• Add setDataSource(source) to GSplatRenderer base class — updates workBuffer and notifies derived classes of format changes
• Override setDataSource in GSplatComputeLocalRenderer to store an independent _dataSource reference, fully decoupling from the work buffer
• Replace all direct this.workBuffer access in the compute renderer with this._dataSource
• Update GSplatManager.handleFormatChange to use setDataSource instead of direct property assignment
• Allow null workBuffer in the base class constructor for standalone compute renderer usage

API Changes:

• GSplatRenderer.setDataSource(source) — new method to set the data source for format and texture access
• GSplatComputeLocalRenderer.setDataSource(source) — override that accepts any { format: GSplatFormat, getTexture: (name) => Texture } object
• The compute renderer can now be constructed without a work buffer and used standalone with a custom data source