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.frag

@@ -0,0 +1,22 @@
+#ifdef GL_ES
+#ifdef GL_FRAGMENT_PRECISION_HIGH
+precision highp float;
+#else
+precision mediump float;
+#endif
+#endif
+
+uniform sampler2D tex;
+in vec2 tex_pos;
+
+#ifdef HIGHER_THAN_300
+out vec4 fragColor;
+#define gl_FragColor fragColor
+#endif
+#ifdef GL_ES
+#define texture texture2D
+#endif
+
+void main() {
+    gl_FragColor = texture(tex, tex_pos);
+}

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;
+}