[VDS] Cache mana symbol renders and skip redundant resizes (#7167)

* [VDS] Cache mana symbol renders and skip redundant resizes

- Render each mana symbol once at a bounded master size and derive every
  requested size from the cached master, avoiding repeated full-size SVG
  rasterization on the GUI thread
- Share scaled results through a process-wide cache keyed by symbol and
  size, so repeated widget creation and rescales don't redo the work
- Skip redundant resize work in ColorIdentityWidget and ManaSymbolWidget
  when sizes did not change

Took 8 minutes


Took 50 seconds

* Move to pixmap generator

Took 8 minutes

Took 4 seconds

---------

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

cockatrice/src/main.cpp

@@ -390,6 +390,7 @@ int main(int argc, char *argv[])
     PingPixmapGenerator::clear();
     CountryPixmapGenerator::clear();
     UserLevelPixmapGenerator::clear();
+    ManaSymbolPixmapGenerator::clear();
 
     return ret;
 }