[`vello_hybrid`] Enable direct textures for image rendering

[grebmeg]

In vello_hybrid, image rendering is currently supported through an atlas texture, which requires uploading images into that atlas before they can be used. We’d like to extend this by enabling direct usage of external textures, allowing them to be sampled directly in addition to the atlas approach.

Our primary goal is to support rendering thousands of images efficiently, and the atlas remains a simple and robust choice for enabling this across native platforms and the web (webgpu and webgl2). Looking ahead, bindless resources appear to be the preferred path for native platforms, though this option is not available in webgl2.