Purge cached text sprites before releasing a font

[botoddly]

Problem

FontSystem.ReleaseFont closes the native TTF_Font and frees its native data, but cached text sprites keyed by that Font remain in the text sprite cache. The cache key keeps the released Font object reachable.

If a cached entry later becomes unreachable and is removed, requesting the same text and released Font again misses the cache and RenderTextToSurface calls SDL_ttf with the closed native font pointer. This makes the public Font.Dispose path capable of reaching a use-after-release native handle.

Scope

• Remove every cached text-sprite entry keyed by a Font before closing that font.
• Invalidate any surviving borrowed texture handles before releasing their backing textures.
• Ensure subsequent text creation with a released Font fails predictably before entering SDL_ttf.
• Keep disposal idempotent.

Tests

• Releasing one font removes only text sprites belonging to that font.
• Cached sprites belonging to other fonts remain valid.
• A retained text asset for the released font is invalidated before its backing texture is released.
• Creating or measuring text with a released font throws ObjectDisposedException without calling SDL_ttf.
• Releasing the same font more than once is safe.