Support Extend::None (decal) for images in vello, vello_cpu and vello_hybrid

[nicoburns]

Summary

Implements peniko::Extend::None (Skia's "decal" tile mode: transparent outside the image) for image sampling in all three renderers, enabling native CSS background-repeat: no-repeat without renderer-side clipping workarounds. Depends on DioxusLabs/peniko#1; Cargo.toml pins peniko to that commit (rev = "4409521...").

Core sampling rule, applied uniformly:

• Coordinates are still clamped for safe texture/memory access (Pad and None share the clamp path).
• Transparency is applied via per-tap validity masks/weights computed from the unclamped coordinate: nearest zeroes the whole sample when out of bounds; bilinear/bicubic zero only the out-of-range taps, so valid edge taps still contribute (no edge-color smearing into the transparent region).

Per renderer:

• vello_cpu: in_bounds_mask/mask_to_weights/apply_mask helpers in fine/common/image.rs, used by the nearest/bilinear/bicubic painters in both the f32 and u8 (lowp) kernels, including the axis-aligned fast paths.
• classic vello: fine.wgsl gains EXTEND_NONE, extend_mode_unclamped, and tap_weight; nearest/bilinear/bicubic image sampling multiply taps by decal weights. The existing 2-bit extend fields in the packed encoding already fit value 3, so no encoding change.
• vello_hybrid: same treatment in render.wesl for the atlas and external-texture paths (nearest/bilinear/bicubic).

EncodedImage::may_have_transparency is now forced when either axis uses Extend::None, so the sparse-strips renderers composite instead of overwriting the background (an Extend::None fill is inherently non-opaque outside the image).

Gradients don't get new behavior: Extend::None on a gradient falls back to Pad.

New snapshot tests: image_none_x_none_y{,_bilinear,_bicubic}, image_none_x_repeat_y, image_repeat_x_none_y, passing on all CPU variants and hybrid. (Running the hybrid tests locally requires mesa-vulkan-drivers/lavapipe, matching CI.)

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/bfa7c4cb42024f28989602f3e38728aa
Requested by: @nicoburns

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