Prevent ScrollArea contents from exceeding the container size (emil…

…k#5006)

When a `ScrollArea` is added to a `Ui` or its contents change
dynamically, the contents will briefly escape the container. This occurs
because `ScrollArea` internally maintains `content_is_too_large` flags,
from which it determines when to clip. The `content_is_too_large` flags
are calculated after painting, so they always lag one frame behind. This
can lead to flickering.

To fix this, I have changed the `ScrollArea` so that it always clips
scrollable content. I believe that this should fix things without
negatively impacting other behavior. To see this, consider how
`ScrollArea` calculates the `content_is_too_large` flag:

```rust
// This calculates a new inner rect, after painting, from the initial clip rect
let inner_rect = {
  // At this point this is the available size for the inner rect.
  let mut inner_size = inner_rect.size();
  
  for d in 0..2 {
      inner_size[d] = match (scroll_enabled[d], auto_shrink[d]) {
          (true, true) => inner_size[d].min(content_size[d]), // shrink scroll area if content is small
          (true, false) => inner_size[d], // let scroll area be larger than content; fill with blank space
          (false, true) => content_size[d], // Follow the content (expand/contract to fit it).
          (false, false) => inner_size[d].max(content_size[d]), // Expand to fit content
      };
  }
  
  Rect::from_min_size(inner_rect.min, inner_size)
};

let outer_rect = Rect::from_min_size(inner_rect.min, inner_rect.size() + current_bar_use);

let content_is_too_large = Vec2b::new(
  scroll_enabled[0] && inner_rect.width() < content_size.x,
  scroll_enabled[1] && inner_rect.height() < content_size.y,
);
```
If `scroll_enabled[d] == true`, then the actual `inner_rect` (which is
calculated after painting contents) will always be smaller than the
original `inner_rect`. Hence, it is safe to unconditionally clip the
contents to `inner_rect` whenever `scroll_enabled[d] == true`.

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* Closes <emilk#4742>
* [x] I have followed the instructions in the PR template

crates/egui/src/containers/scroll_area.rs

@@ -572,10 +572,8 @@ impl ScrollArea {
             let mut content_clip_rect = ui.clip_rect();
             for d in 0..2 {
                 if scroll_enabled[d] {
-                    if state.content_is_too_large[d] {
-                        content_clip_rect.min[d] = inner_rect.min[d] - clip_rect_margin;
-                        content_clip_rect.max[d] = inner_rect.max[d] + clip_rect_margin;
-                    }
+                    content_clip_rect.min[d] = inner_rect.min[d] - clip_rect_margin;
+                    content_clip_rect.max[d] = inner_rect.max[d] + clip_rect_margin;
                 } else {
                     // Nice handling of forced resizing beyond the possible:
                     content_clip_rect.max[d] = ui.clip_rect().max[d] - current_bar_use[d];