Introduce dithering to reduce banding (#4497)

This PR introduces dithering in the egui_glow and egui_wgpu backends to
reduce banding artifacts.

It's based on the approach mentioned in #4493 with the small difference
that the amount of noise is scaled down slightly to avoid dithering
colors that can be represented exactly. This keeps flat surfaces clean.

Exaggerated dithering to show what is happening:
![Screenshot from 2024-05-14
19-09-48](https://github.com/emilk/egui/assets/293536/75782b83-9023-4cb2-99f7-a24e15fdefcc)

Subtle dithering as commited.
![Screenshot from 2024-05-14
19-13-40](https://github.com/emilk/egui/assets/293536/eb904698-a6ec-494a-952b-447e9a49bfda)

Closes #4493

crates/egui_glow/src/painter.rs

@@ -138,6 +138,7 @@ impl Painter {
         gl: Arc<glow::Context>,
         shader_prefix: &str,
         shader_version: Option<ShaderVersion>,
+        dithering: bool,
     ) -> Result<Self, PainterError> {
         crate::profile_function!();
         crate::check_for_gl_error_even_in_release!(&gl, "before Painter::new");
@@ -197,9 +198,10 @@ impl Painter {
                 &gl,
                 glow::FRAGMENT_SHADER,
                 &format!(
-                    "{}\n#define NEW_SHADER_INTERFACE {}\n#define SRGB_TEXTURES {}\n{}\n{}",
+                    "{}\n#define NEW_SHADER_INTERFACE {}\n#define DITHERING {}\n#define SRGB_TEXTURES {}\n{}\n{}",
                     shader_version_declaration,
                     shader_version.is_new_shader_interface() as i32,
+                    dithering as i32,
                     srgb_textures as i32,
                     shader_prefix,
                     FRAG_SRC