Put svg rendering behind a feature gate

This reduces binary size when SVG supoprt is not needed.

Cargo.toml

@@ -14,6 +14,8 @@ categories = ["gui"]
 [features]
 # Enables a debug view in native platforms (press F12)
 debug = ["iced_winit/debug"]
+# Enables support for SVG rendering
+svg = ["iced_wgpu/svg"]
 
 [badges]
 maintenance = { status = "actively-developed" }