Add Options::reduce_texture_memory to free up RAM (emilk#4431)

## Summary

This PR introduces a new configuration option `reduce_texture_memory` in
`egui`.

## Changes

- Added `reduce_texture_memory` option in `egui`. When set to `true`,
`egui` will discard the loaded image data after the texture is uploaded
to the GPU, reducing memory usage. This is beneficial when handling a
large number of images and retaining the image data is unnecessary,
potentially saving substantial memory. However, this makes it impossible
to serialize the loaded images or render on non-GPU systems. Default is
`false`.

## Impact

This new configuration option gives users more control over their memory
usage, especially when dealing with a large number or large resolution
of images. It allows users to optimize their applications based on their
specific needs and constraints.

crates/egui/src/load/texture_loader.rs

@@ -28,6 +28,17 @@ impl TextureLoader for DefaultTextureLoader {
                     let handle = ctx.load_texture(uri, image, texture_options);
                     let texture = SizedTexture::from_handle(&handle);
                     cache.insert((uri.into(), texture_options), handle);
+                    let reduce_texture_memory = ctx.options(|o| o.reduce_texture_memory);
+                    if reduce_texture_memory {
+                        let loaders = ctx.loaders();
+                        loaders.include.forget(uri);
+                        for loader in loaders.bytes.lock().iter().rev() {
+                            loader.forget(uri);
+                        }
+                        for loader in loaders.image.lock().iter().rev() {
+                            loader.forget(uri);
+                        }
+                    }
                     Ok(TexturePoll::Ready { texture })
                 }
             }

crates/egui/src/memory.rs

@@ -235,6 +235,19 @@ pub struct Options {
 
     /// Controls the speed at which we zoom in when doing ctrl/cmd + scroll.
     pub scroll_zoom_speed: f32,
+
+    /// If `true`, `egui` will discard the loaded image data after
+    /// the texture is loaded onto the GPU to reduce memory usage.
+    ///
+    /// In modern GPU rendering, the texture data is not required after the texture is loaded.
+    ///
+    /// This is beneficial when using a large number or resolution of images and there is no need to
+    /// retain the image data, potentially saving a significant amount of memory.
+    ///
+    /// The drawback is that it becomes impossible to serialize the loaded images or render in non-GPU systems.
+    ///
+    /// Default is `false`.
+    pub reduce_texture_memory: bool,
 }
 
 impl Default for Options {
@@ -260,6 +273,7 @@ impl Default for Options {
             // Input:
             line_scroll_speed,
             scroll_zoom_speed: 1.0 / 200.0,
+            reduce_texture_memory: false,
         }
     }
 }
@@ -279,6 +293,7 @@ impl Options {
 
             line_scroll_speed,
             scroll_zoom_speed,
+            reduce_texture_memory,
         } = self;
 
         use crate::Widget as _;
@@ -297,6 +312,8 @@ impl Options {
                 );
 
                 ui.checkbox(warn_on_id_clash, "Warn if two widgets have the same Id");
+
+                ui.checkbox(reduce_texture_memory, "Reduce texture memory");
             });
 
         use crate::containers::*;

crates/egui_demo_app/src/apps/image_viewer.rs

@@ -184,6 +184,11 @@ impl eframe::App for ImageViewer {
             ui.add_space(5.0);
             ui.label("Aspect ratio is maintained by scaling both sides as necessary");
             ui.checkbox(&mut self.maintain_aspect_ratio, "Maintain aspect ratio");
+
+            // forget all images
+            if ui.button("Forget all images").clicked() {
+                ui.ctx().forget_all_images();
+            }
         });
 
         egui::CentralPanel::default().show(ctx, |ui| {