Add opacity factor to `TextShape` (#3916)

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This PR simply allows you to override the opacity of a Galley when you
draw it on screen. Last year I opened #3548 and some changes were
requested to the PR, but unfortunately school got really busy and I
wasn't able to apply them. This PR supersedes #3548 and applys the
changes requested in that PR

Closes #3548

crates/epaint/src/shape.rs

@@ -745,6 +745,10 @@ pub struct TextShape {
     /// This only affects the glyphs and will NOT replace background color nor strikethrough/underline color.
     pub override_text_color: Option<Color32>,
 
+    /// If set, the text will be rendered with the given opacity in gamma space
+    /// Affects everything: backgrounds, glyphs, strikethough, underline, etc.
+    pub opacity_factor: f32,
+
     /// Rotate text by this many radians clockwise.
     /// The pivot is `pos` (the upper left corner of the text).
     pub angle: f32,
@@ -762,6 +766,7 @@ impl TextShape {
             underline: Stroke::NONE,
             fallback_color,
             override_text_color: None,
+            opacity_factor: 1.0,
             angle: 0.0,
         }
     }
@@ -792,6 +797,13 @@ impl TextShape {
         self.angle = angle;
         self
     }
+
+    /// Render text with this opacity in gamma space
+    #[inline]
+    pub fn with_opacity_factor(mut self, opacity_factor: f32) -> Self {
+        self.opacity_factor = opacity_factor;
+        self
+    }
 }
 
 impl From<TextShape> for Shape {

crates/epaint/src/shape_transform.rs

@@ -56,6 +56,7 @@ pub fn adjust_colors(shape: &mut Shape, adjust_color: &impl Fn(&mut Color32)) {
             underline,
             fallback_color,
             override_text_color,
+            opacity_factor: _,
             angle: _,
         }) => {
             adjust_color(&mut underline.color);

crates/epaint/src/tessellator.rs

@@ -1474,6 +1474,7 @@ impl Tessellator {
             underline,
             override_text_color,
             fallback_color,
+            opacity_factor,
             angle,
         } = text_shape;
 
@@ -1481,6 +1482,10 @@ impl Tessellator {
             return;
         }
 
+        if *opacity_factor <= 0.0 {
+            return;
+        }
+
         if galley.pixels_per_point != self.pixels_per_point {
             eprintln!("epaint: WARNING: pixels_per_point (dpi scale) have changed between text layout and tessellation. \
                        You must recreate your text shapes if pixels_per_point changes.");
@@ -1548,6 +1553,10 @@ impl Tessellator {
                             color = *fallback_color;
                         }
 
+                        if *opacity_factor < 1.0 {
+                            color = color.gamma_multiply(*opacity_factor);
+                        }
+
                         crate::epaint_assert!(color != Color32::PLACEHOLDER, "A placeholder color made it to the tessellator. You forgot to set a fallback color.");
 
                         let offset = if *angle == 0.0 {