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/shader/common/image.vert

@@ -0,0 +1,9 @@
+uniform mat4 u_Transform;
+
+in vec2 i_Position;
+out vec2 tex_pos;
+
+void main() {
+    gl_Position = u_Transform * vec4(i_Position, 0.0, 1.0);
+    tex_pos = i_Position;
+}