Add Options::reduce_texture_memory to free up RAM (#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_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| {