Add image/svg support to iced_glow

https://github.com/iced-rs/iced/issues/674

Uses image/svg support in `iced_graphics`. The is not currently using an
atlas, and uses one texture/draw per image. This should be good enough
for now; supporting images with glow is better than not supporting them,
and if something else performs better, that improvement can be made
without any change to the public API.

glow/src/lib.rs

@@ -24,6 +24,8 @@
 pub use glow;
 
 mod backend;
+#[cfg(any(feature = "image_rs", feature = "svg"))]
+mod image;
 mod program;
 mod quad;
 mod text;