Add image drawing, text rotation, pie/arc paths, and dash patterns to VectorCanvas

[pgourlain]

Summary

This PR extends the drawing surface with several features that were previously
missing, fixes a PNG decoding gap, and removes a vulnerable transitive dependency.

Added

• Image drawing — PNG and JPEG images can now be placed at a given position
  on a VectorCanvas, loaded from a file path, a byte array, or a stream.
  Five layout modes are supported: Stretch, Contain, centred Cover,
  top-left Cover, and a natural-size top-left crop. Clipping applied while
  drawing an image is scoped to that operation and does not leak into
  subsequent canvas commands.
• Text rotation — text can optionally be rotated clockwise around its
  baseline point. Works with both the standard fonts and registered custom fonts.
• Elliptical arcs and pies — PathDescriptor gained elliptical arcs and
  closed sectors, with FillPie, StrokePie, and DrawPie exposed as
  canvas-level conveniences.
• Dash patterns — lines and stroked rectangles now accept native dash
  patterns and a dash phase, instead of being limited to solid strokes.

Fixed

• PNG decoding now handles 8-bit grayscale images with an alpha channel
  (colour type 4),