In the script's API `COMPANY_INVALID` has a value of -1, whereas the internal
game's `INVALID_COMPANY` has a value of 255. Since the script's API also has
a `COMPANY_SPECTATOR` with a value of 255, these enumerations cannot be easily
reconciled by casting. As such, replace all casts in the script API with
either ScriptCompany::FromScriptCompanyID or ScriptCompany::ToScriptCompanyID.
Also make clear whether CompanyID is ::CompanyID or ScriptCompany::CompanyID
by using either one of those over CompanyID in the script's API.
When a game script is in company mode, it pretends to be another company. When
that company disappear (bankruptcy/merger), the game script still uses that
company and it keeps calling functions as if it is that company.
For example, ScriptEngine::IsBuildable internally dereferences Company without
checks, causing a null dereference for any ScriptEngine function when called
from a company scope of a company that has disappeared.
Guard against this by extending the ScriptCompanyScope::IsValid check to also
check for the company still being active.
