Make image Cache eviction strategy less aggressive in iced_wgpu

Instead of trimming unconditionally at the end of
a frame, we now trim the cache only when there is a
cache miss.

This way, images that are not visible but still a
part of the layout will stay cached. Eviction will
only happen when the images are not a part of the
UI for two consectuive frames.

wgpu/src/image/raster.rs

@@ -40,6 +40,7 @@ impl Memory {
 pub struct Cache {
     map: FxHashMap<u64, Memory>,
     hits: FxHashSet<u64>,
+    should_trim: bool,
 }
 
 impl Cache {
@@ -55,6 +56,8 @@ impl Cache {
             Err(_) => Memory::Invalid,
         };
 
+        self.should_trim = true;
+
         self.insert(handle, memory);
         self.get(handle).unwrap()
     }
@@ -86,6 +89,11 @@ impl Cache {
 
     /// Trim cache misses from cache
     pub fn trim(&mut self, atlas: &mut Atlas) {
+        // Only trim if new entries have landed in the `Cache`
+        if !self.should_trim {
+            return;
+        }
+
         let hits = &self.hits;
 
         self.map.retain(|k, memory| {
@@ -101,6 +109,7 @@ impl Cache {
         });
 
         self.hits.clear();
+        self.should_trim = false;
     }
 
     fn get(&mut self, handle: &image::Handle) -> Option<&mut Memory> {