Add Canvas::set_clip_path — anti-aliased arbitrary-path clipping on both backends

Add `Canvas::set_clip_path(&[PathCmd])`, clipping subsequent draws to an arbitrary vector path with an anti-aliased edge, on both the software and GLES backends. It complements the existing rect clip (`set_clip_rects`) and is what an embedder needs to render a shaped clip — a circular avatar, a rounded card, a `VectorDrawable` mask — rather than a bounding box. Kept general rather than tied to any one consumer.
Software backend: rasterise the path into an anti-aliased tiny-skia coverage `Mask` (Winding fill) and install it as the active clip mask. Every software primitive already threads `clip_mask` through tiny-skia (fills, strokes, lines, paths, images, text, blit), so the path clip applies uniformly with smooth edges. `clip_bounds` reports the path's bounding box while it is active.
GLES backend: a 1-bit stencil would clip exactly but leave a hard, aliased edge, so instead the clipped draws are captured into an offscreen layer and composited back through an anti-aliased coverage mask. `set_clip_path` rasterises the path coverage (tiny-skia, anti-aliased), uploads it as a mask texture, allocates a full-canvas layer FBO on first use, and redirects subsequent draws to it via `activate_target`. Ending the clip (`clear_clip` / `set_clip_rects` / a new `set_clip_path`) composites the layer back onto the canvas FBO with a new two-sampler program (`CLIP_COMPOSITE_FRAG_SRC`) that multiplies the layer colour by the mask coverage and blends it premultiplied-over. The layer attaches to the canvas's own shadow FBO, so it needs no stencil bits in the EGL config; it is freed and reallocated on resize and freed on drop, and shared programs/uniforms are copied to sub-canvases like the rest.
Usage: a path clip is bracketed — `set_clip_path` then, after the clipped draws, `clear_clip` or `set_clip_rects` to flush it (on GLES this is when the layer is composited). Snapshot the prior clip with `clip_bounds` beforehand and restore it with `set_clip_rects` to compose with an outer clip without leaking state.
Add an `examples/clip_path.rs` demo (rounded rect, circle, triangle — same smooth result on both backends) and software-backend unit tests covering the bounding box, the empty-path clear, and a pixel-level check that a triangular clip masks a fill to the path silhouette rather than its bounding box. The GLES layer-composite path needs a live GL context and is exercised by the example.
Also fix three rustdoc intra-doc-link warnings surfaced along the way: a private-item link in `app.rs` (`scroll`) and the new GLES doc (`SoftwareCanvas::set_clip_path`) demoted to code spans, and a redundant explicit link target in `chassis.rs`.

src/gles_render/shaders.rs

@@ -147,6 +147,30 @@ void main()
 }
 "##;
 
+// Clip-layer composite shader for `set_clip_path`. Path-clipped content is
+// drawn to an offscreen layer; this composites that layer onto the canvas
+// multiplied by an anti-aliased coverage mask, giving a smooth clipped edge.
+// `u_layer` is the full-canvas layer texture (premultiplied), `u_mask` the
+// path coverage rasterised over `u_bbox` (x, y, w, h in canvas pixels). The
+// quad is drawn over the bbox; `v_uv` runs 0..1 across it. Both FBO-backed
+// textures are sampled y-flipped (GL's lower-left origin).
+pub( super ) const CLIP_COMPOSITE_FRAG_SRC: &str = r##"
+precision mediump float;
+varying vec2 v_uv;
+uniform sampler2D u_layer;
+uniform sampler2D u_mask;
+uniform vec2 u_canvas;
+uniform vec4 u_bbox;
+void main()
+{
+	vec2 sp  = u_bbox.xy + v_uv * u_bbox.zw;
+	vec2 luv = vec2( sp.x / u_canvas.x, 1.0 - sp.y / u_canvas.y );
+	vec4 col = texture2D( u_layer, luv );
+	float cov = texture2D( u_mask, vec2( v_uv.x, 1.0 - v_uv.y ) ).a;
+	gl_FragColor = col * cov;
+}
+"##;
+
 // Fragment shader for single-channel glyph textures with color tint.
 //
 // Glyphs live in a shared GL_LUMINANCE atlas. `u_uv_scale` and `u_uv_offset`