gpui_wgpu: Fix fallback fonts disappearing from the font database on Linux (#65269)

Closes #61660

On Linux, `load_family` skips any font without an `m` glyph, since the
editor uses the width of `m` for layout and would panic otherwise. That
part's fine. The problem is it also deleted the font from the fontdb
database, and has since the cosmic-text port (#7539).

That hurts because fonts like Arabic, Hebrew, Devanagari, Noto Color
Emoji and Symbols Nerd Font have no `m`, and they're exactly what
cosmic-text falls back to for glyphs your main font doesn't have. So if
one of them ever went through `load_family`, say because you put it in
`buffer_font_fallbacks`, it was gone for the rest of the session and
that script showed up as empty boxes.

It also blocks #64199, where the font picker renders each row in its own
font. On Linux that would call `load_family` on every installed font and
wipe out all the non-Latin and emoji ones. Running the Linux text system
against the fonts on my Mac, 334 of 806 faces got removed.

This keeps the `m` check and drops the removal. You still can't use
these as your buffer or UI font, but fallback works again.

---

Tested on Ubuntu 24.04. 

By default, emoji render because cosmic-text finds Noto Color Emoji on
its own:
<img width="400" alt="control-no-fallbacks"
src="https://github.com/user-attachments/assets/ac9b5aa1-fb06-4033-9ca1-1f4938b34839"
/>

Setting `"buffer_font_fallbacks": ["Noto Color Emoji"]` sends the font
through `load_family`, which deleted it from the database since it has
no `m`, so emoji turned into empty boxes:
<img width="400" alt="before"
src="https://github.com/user-attachments/assets/0af20580-f074-4522-a4b2-9c0a1312594f"
/>

With this PR the font stays in the database and emoji render again:
<img width="400" alt="after"
src="https://github.com/user-attachments/assets/a1fe9e91-dc8e-413d-a89f-d0968c410a6e"
/>

Release Notes:

- Fixed emoji, symbol and non-Latin fonts rendering as empty boxes on
Linux after being used as a font fallback.
- [GPUI] Fixed fonts without an `m` glyph being removed from the font
database on Linux (`gpui_wgpu`) when loaded by family name, which broke
fallback for emoji, symbol and non-Latin text.

crates/gpui_wgpu/src/cosmic_text_system.rs

@@ -418,10 +418,17 @@ impl CosmicTextSystemState {
                 "Segoe Fluent Icons",
             ];
 
+            // This only decides whether a face can be a primary font. The
+            // editor measures layout with the width of 'm' and would panic
+            // without it.
+            //
+            // Don't remove the face from the font database. Arabic, Devanagari
+            // and emoji fonts often have no Latin glyphs, and they are exactly
+            // what cosmic-text falls back to when the primary font lacks a
+            // glyph.
             if font.as_swash().charmap().map('m') == 0
                 && !allowed_bad_font_names.contains(&postscript_name.as_str())
             {
-                self.font_system.db_mut().remove_face(font.id());
                 continue;
             };