Fix overflow panic when filling paths with extreme coordinates

[StefanoD]

When anti-aliased filling a path whose device-space vertical bounds lie far outside the clip, start_y/stop_y (the path bounds) were shifted up for supersampling with a plain <<. A large-negative bound then overflowed i32 and wrapped into a large-positive value, escaping the subsequent clip clamp and breaking walk_edges' last_y >= curr_y invariant.

Use a saturating shift so out-of-range bounds stay on the correct side and get clamped to the clip as intended. In-range bounds are unaffected.

Originally reported against resvg: linebender/resvg#933

Reproduced with:

<svg height="32" viewBox="0 0 8.4.9" width="32" x="w">
  <g m=""><g><path d="m3 6h8v-158758583"/></g></g>
</svg>

Crash dump (debug build):

thread 'main' panicked at tiny-skia/src/scan/path.rs:221:13:
assertion failed: edges[curr_idx].last_y >= curr_y as i32
   3: tiny_skia::scan::path::walk_edges
   4: tiny_skia::scan::path::fill_path_impl
   5: tiny_skia::scan::path_aa::fill_path_impl
   6: tiny_skia::scan::path_aa::fill_path
   7: tiny_skia::painter::<impl tiny_skia::pixmap::PixmapMut>::fill_path

Crash dump (release build):

thread 'main' panicked at tiny-skia/src/pipeline/mod.rs:181:31:
range start index 15006310411 out of range for slice of length 1024
   3: tiny_skia::pipeline::lowp::load_dst_tail
   4: tiny_skia::pipeline::lowp::start
   5: <tiny_skia::pipeline::blitter::RasterPipelineBlitter as tiny_skia::blitter::Blitter>::blit_anti_h
   6: tiny_skia::scan::path_aa::SuperBlitter::flush
   7: tiny_skia::scan::path_aa::fill_path_impl

Generated by Claude

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Note: I accidentally closed the original PR (#178) by deleting my fork, which auto-closed it. This reopens the same change — the branch and commits are unchanged.