Expand max font atlas size from 8k to 16k (#5257)

When using fonts with an average of 50,000 characters,
'epaint texture atlas overflowed!' may be printed and cause problems.
It is necessary to expand the max value related to texture.

* Closes #5256

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/epaint/src/texture_atlas.rs

@@ -159,8 +159,8 @@ impl TextureAtlas {
     }
 
     fn max_height(&self) -> usize {
-        // the initial width is likely the max texture side size
-        self.image.width()
+        // the initial width is set to the max size
+        self.image.height().max(self.image.width())
     }
 
     /// When this get high, it might be time to clear and start over!