Skia: hold image Blob alongside cached image shader

[nicoburns]

Summary

shader_from_image_brush builds the Skia image via unsafe { SkData::new_bytes(...) }, which references the peniko Blob's pixel bytes without owning them. The resulting shader is stored in the generational image_shader cache, which outlives the set_paint_brush call — so if the caller drops the ImageData after painting, the cached shader dangles (UB).

Fix: change the cache value from Shader to (Shader, peniko::Blob<u8>) and store a clone of the image blob (a cheap Arc clone) alongside the shader, tying the pixel data's lifetime to the cache entry.

Note: the cache key still ignores brush_transform/sampler (audit item 1.2); that's a separate issue not addressed here.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/b0ca09237beb462e9ea6b7be54018019
Requested by: @nicoburns

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