Add path renderer for arcs/strokes/dashing/regions

[jserv]

This introduces src/path/ as an in-tree scanline path renderer (builder, quad/cubic flatten, arc-to-cubic, stroke expansion with proper miter / bevel / round joins and butt/round/projecting caps, scanline rasterizer, pixman region builder). Route through it:

• XDrawArc/XFillArc/XDrawArcs/XFillArcs for ArcChord, dashed, wide, or large solid arcs. Small LineSolid one-pixel arcs stay on the legacy point renderer to avoid regressing tiny primitive overhead.
• XDrawLine/XDrawLines/XDrawSegments/XDrawRectangle wide-stroke and LineOnOffDash/LineDoubleDash paths, with the X11 default miter limit (10) and dash phase carried across MoveTo within a call.
• XPolygonRegion edge/span construction with pixman union storage.

Other coverage gaps closed:

• Implement XDrawPoints and XDrawRectangles (previously stubs).
• Add non-GXcopy software fallback for line, segment, polyline, and rectangle batches so GXxor/GXinvert/plane-mask take effect; batched per line bounding box instead of round-tripping per pixel.
• XDrawRectangle now traces the spec-mandated (w+1) by (h+1) outline with no double-XOR at corners.
• Closed-polyline strokes emit joins at the closure vertex instead of caps, so wide rectangle outlines look right.

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Summary by cubic

Adds an in-tree scanline path renderer and routes arcs, wide/dashed strokes, and region ops through it for correct joins/caps, better spec compliance, and faster batched draws. Also improves GC raster ops and implements missing point/rectangle APIs.

• New Features

  • New src/path/ renderer: builder, adaptive flattening, arc-to-cubic, stroke expansion (miter/bevel/round; butt/round/projecting caps), scanline rasterizer, and pixman span/region composition.
  • Routes XDrawArc/XFillArc/XDrawArcs/XFillArcs for ArcChord, wide strokes, dashing, and large solid arcs; tiny 1px LineSolid arcs stay on the legacy path.
  • Wide and dashed strokes for XDrawLine/XDrawLines/XDrawSegments/XDrawRectangle with miter limit 10; dash phase preserved across MoveTo.
  • XPolygonRegion built from edges/spans with pixman storage.
  • Implemented XDrawPoints and XDrawRectangles.

• Bug Fixes

  • GC raster functions (GXxor, GXinvert, plane-mask) now apply to batched lines/segments/polylines/rectangles via a software fallback per line bbox.
  • XDrawRectangle now traces the spec-mandated (w+1)×(h+1) outline with no double XOR at corners.
  • Closed polylines emit a join at the closure vertex, fixing wide rectangle outlines.

^{Written for commit 21f55fc. Summary will update on new commits.}