Don't panic when replacement glyph is not found (#4542)

I wanted to implement a font picker that loads all system fonts but ran
into panics due to missing glyphs. Falling back to an empty glyph when
none of the fallback glyphs are available avoids the panic.

crates/epaint/src/text/font.rs

@@ -370,9 +370,11 @@ impl Font {
             .glyph_info_no_cache_or_fallback(PRIMARY_REPLACEMENT_CHAR)
             .or_else(|| slf.glyph_info_no_cache_or_fallback(FALLBACK_REPLACEMENT_CHAR))
             .unwrap_or_else(|| {
-                panic!(
-                    "Failed to find replacement characters {PRIMARY_REPLACEMENT_CHAR:?} or {FALLBACK_REPLACEMENT_CHAR:?}"
-                )
+                #[cfg(feature = "log")]
+                log::warn!(
+                    "Failed to find replacement characters {PRIMARY_REPLACEMENT_CHAR:?} or {FALLBACK_REPLACEMENT_CHAR:?}. Will use empty glyph."
+                );
+                (0, GlyphInfo::default())
             });
         slf.replacement_glyph = replacement_glyph;