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/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 {