[example_web] show loading of an image

Required some redesign of `TextureAllocator` as well as
some improvements to the fetch API.

egui/src/app.rs

@@ -77,12 +77,19 @@ pub struct AppOutput {
 }
 
 pub trait TextureAllocator {
-    /// Allocate a user texture (EXPERIMENTAL!)
-    fn new_texture_srgba_premultiplied(
+    /// A.locate a new user texture.
+    fn alloc(&mut self) -> crate::TextureId;
+
+    /// Set or change the pixels of a user texture.
+    fn set_srgba_premultiplied(
         &mut self,
+        id: crate::TextureId,
         size: (usize, usize),
-        pixels: &[crate::Srgba],
-    ) -> crate::TextureId;
+        srgba_pixels: &[crate::Srgba],
+    );
+
+    /// Free the given texture.
+    fn free(&mut self, id: crate::TextureId);
 }
 
 /// A place where you can store custom data in a way that persists when you restart the app.

egui/src/demos/color_test.rs

@@ -41,7 +41,7 @@ impl ColorTest {
         ui.label("Use a color picker to ensure this color is (255, 165, 0) / #ffa500");
         ui.wrap(|ui| {
             ui.style_mut().spacing.item_spacing.y = 0.0; // No spacing between gradients
-            let g = Gradient::one_color(Srgba::new(255, 165, 0, 255));
+            let g = Gradient::one_color(Srgba::from_rgb(255, 165, 0));
             self.vertex_gradient(ui, "orange rgb(255, 165, 0) - vertex", WHITE, &g);
             self.tex_gradient(
                 ui,
@@ -125,7 +125,7 @@ impl ColorTest {
             ui,
             tex_allocator,
             RED,
-            (TRANSPARENT, Srgba::new(0, 0, 255, 0)),
+            (TRANSPARENT, Srgba::from_rgba_premultiplied(0, 0, 255, 0)),
         );
 
         ui.separator();
@@ -367,7 +367,9 @@ impl TextureManager {
             let pixels = gradient.to_pixel_row();
             let width = pixels.len();
             let height = 1;
-            tex_allocator.new_texture_srgba_premultiplied((width, height), &pixels)
+            let id = tex_allocator.alloc();
+            tex_allocator.set_srgba_premultiplied(id, (width, height), &pixels);
+            id
         })
     }
 }

egui/src/paint/color.rs

@@ -24,15 +24,41 @@ impl std::ops::IndexMut<usize> for Srgba {
     }
 }
 
+// TODO: remove ?
 pub const fn srgba(r: u8, g: u8, b: u8, a: u8) -> Srgba {
-    Srgba::new(r, g, b, a)
+    Srgba::from_rgba_premultiplied(r, g, b, a)
 }
 
 impl Srgba {
+    #[deprecated = "Use from_rgb(..), from_rgba_premultiplied(..) or from_srgba_unmultiplied(..)"]
     pub const fn new(r: u8, g: u8, b: u8, a: u8) -> Self {
         Self([r, g, b, a])
     }
 
+    pub const fn from_rgb(r: u8, g: u8, b: u8) -> Self {
+        Self([r, g, b, 255])
+    }
+
+    pub const fn from_rgb_additive(r: u8, g: u8, b: u8) -> Self {
+        Self([r, g, b, 0])
+    }
+
+    /// From `sRGBA` with premultiplied alpha.
+    pub const fn from_rgba_premultiplied(r: u8, g: u8, b: u8, a: u8) -> Self {
+        Self([r, g, b, a])
+    }
+
+    /// From `sRGBA` WITHOUT premultiplied alpha.
+    pub fn from_rgba_unmultiplied(r: u8, g: u8, b: u8, a: u8) -> Self {
+        if a == 255 {
+            Self::from_rgba_premultiplied(r, g, b, a) // common-case optimization
+        } else {
+            Rgba::from(Self::from_rgb(r, g, b))
+                .multiply(a as f32 / 255.0)
+                .into()
+        }
+    }
+
     pub const fn gray(l: u8) -> Self {
         Self([l, l, l, 255])
     }
@@ -476,7 +502,7 @@ fn test_hsv_roundtrip() {
     for r in 0..=255 {
         for g in 0..=255 {
             for b in 0..=255 {
-                let srgba = Srgba::new(r, g, b, 255);
+                let srgba = Srgba::from_rgb(r, g, b);
                 let hsva = Hsva::from(srgba);
                 assert_eq!(srgba, Srgba::from(hsva));
             }