vello_hybrid: Support extend modes for external texture rects

[LaurenzV]

Based on #1812.

Context

The main reason for opening this PR is that currently, the external texture rect API has no way of supporting other extend modes. This PR proposes tweaking the existing API for drawing external texture rects such that this use case can be accommodated.

Implementation

It took me a while to figure out what the best way of implementing this is. Ultimately, I arrived at the conclusion that we should change the existing draw_texture_rects API to instead have a single draw_texture_rect API. I presume the original motivation for having the ability to submit multiple texture rectangles at once is performance. However, if you look at the actual implementation, the bulk of the work happens inside of the loop that iterates over each rectangles. Only very little work is lifted outside of the loop, so I don’t see any performance benefit in having a batched method for this. On the other hand, by making the API only take a single texture rect, we can make it more consistent with how other methods work and more easily add support for the missing features. For example, the Scene::paint_transform field now actually serves a purpose for external textures.

In order to get what we need, I also changed the API to taking two different rectangles: A source and a destination rectangle. The source rectangle defines what the area is we want to sample, and the destination (as well as path/paint transform) how it’s mapped into the actual scene. Any pixel that would fall outside of the source region but still be visible in the destination region is sampled according to the extend mode.

This PR was done with assistance of GPT 5.6 Sol.

Also CC @tomcur since he designed the original API.

[grebmeg]

Could we rename these to source_rect and destination_rect for consistency, since both values are rectangles? src_rect and dst_rect would also be concise.

[LaurenzV]

Yep!

[LaurenzV]

Ended up doing dest instead.