feat: support tiny-skia CPU backend

[zao111222333]

Summary

I introduce PlotRenderStrategy to switch between the existing shader path and a new canvas-based path for CPU
renderers like tiny-skia. The strategy can be configured manually or auto-detected from the active iced graphics
backend. The shared plot update/draw implementation is reused by both shader and canvas programs, and CPU picking is used when rendering through canvas.

I also add examples/auto_backend.rs, which auto-detects the render strategy, displays the current strategy, and
provides x/y layout offset sliders for testing PlotWidget behavior under different backends.

Known issue

When running with:

ICED_BACKEND="tiny-skia" cargo run --example auto_backend

PlotWidget may show corrupted canvas content after applying layout offsets. This is an iced 0.14 tiny-skia/canvas
issue, not a PlotWidget rendering bug.

The issue is already fixed in iced 0.15.0-dev, which can be verified here:

https://github.com/zao111222333/iced_plot/tree/iced-0.15.0-dev

That branch only upgrades iced, and the offset corruption disappears.

[donkeyteethUX]

Thanks for taking this on!

It looks good. All the examples I tried worked fine with tiny-skia except for huge_scatter (which is not surprising). Just a few small requests.

[zao111222333]

Thank you for reviewing! I have made revisions based on your suggestions.