This fixes "out of realized font handles" errors seen with Valve games like Half-Life 2 (which uses more than 1000 font handles).
The new limit of 5000 is roughly inspired by the Windows limit, although there are differences. On Windows 11 I can call `CreateFont()` (and then `SetMapMode()`, `SelectObject()`, `SetTextAlign()` to realize the font) exactly 5000 times before it hits the handle limit.
Under Wine, child fonts use handles as well, so the limit is usually hit before 5000 `CreateFont()` calls. But, there is caching so that `CreateFont()` calls with identical arguments do not count towards the limit. Ultimately the old limit of 256 was sufficient for almost all applications, and 5000 should be enough for all but the worst-behaved apps.
-- v3: win32u: Make font handle table dynamically growable. win32u: Track free font handles through indices instead of pointers. win32u: Enter font_lock in NtGdiMakeFontDir.