add canvas_image builtin for PNG rendering with alpha blending

Registers canvas_image(path, x, y, w, h) in the type system and
implements it in the interpreter using the image crate — scales to
exact dimensions via Lanczos3 and alpha-composites onto the pixmap.

Cargo.toml

@@ -62,3 +62,4 @@ winit       = { version = "0.29", default-features = false, features = ["rwh_05"
 softbuffer  = "0.3"
 tiny-skia   = "0.11"
 fontdue     = "0.8"
+image       = { version = "0.25", default-features = false, features = ["png"] }