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/image/textures.rs

@@ -0,0 +1,82 @@
+use glow::HasContext;
+use iced_graphics::image::{TextureStore, TextureStoreEntry};
+
+#[derive(Debug)]
+pub struct Textures;
+
+impl Textures {
+    pub fn new() -> Self {
+        Self
+    }
+}
+
+impl TextureStore for Textures {
+    type Entry = Entry;
+    type State<'a> = &'a glow::Context;
+
+    fn upload(
+        &mut self,
+        width: u32,
+        height: u32,
+        data: &[u8],
+        gl: &mut &glow::Context,
+    ) -> Option<Self::Entry> {
+        unsafe {
+            let texture = gl.create_texture().expect("create texture");
+            gl.bind_texture(glow::TEXTURE_2D, Some(texture));
+            gl.tex_image_2d(
+                glow::TEXTURE_2D,
+                0,
+                glow::SRGB8_ALPHA8 as i32,
+                width as i32,
+                height as i32,
+                0,
+                glow::BGRA,
+                glow::UNSIGNED_BYTE,
+                Some(data),
+            );
+            gl.tex_parameter_i32(
+                glow::TEXTURE_2D,
+                glow::TEXTURE_WRAP_S,
+                glow::CLAMP_TO_EDGE as _,
+            );
+            gl.tex_parameter_i32(
+                glow::TEXTURE_2D,
+                glow::TEXTURE_WRAP_T,
+                glow::CLAMP_TO_EDGE as _,
+            );
+            gl.tex_parameter_i32(
+                glow::TEXTURE_2D,
+                glow::TEXTURE_MIN_FILTER,
+                glow::LINEAR as _,
+            );
+            gl.tex_parameter_i32(
+                glow::TEXTURE_2D,
+                glow::TEXTURE_MAG_FILTER,
+                glow::LINEAR as _,
+            );
+            gl.bind_texture(glow::TEXTURE_2D, None);
+
+            Some(Entry {
+                size: (width, height),
+                texture,
+            })
+        }
+    }
+
+    fn remove(&mut self, entry: &Entry, gl: &mut &glow::Context) {
+        unsafe { gl.delete_texture(entry.texture) }
+    }
+}
+
+#[derive(Debug)]
+pub struct Entry {
+    size: (u32, u32),
+    pub texture: glow::NativeTexture,
+}
+
+impl TextureStoreEntry for Entry {
+    fn size(&self) -> (u32, u32) {
+        self.size
+    }
+}