Fix biome-specific mappreviews in 7f602037ba for zipped mods, refs #4962.

Add TextureExists to avoid needless redundant hardcoding of the
filenames.
Remove mapBiome.Preview from gamedescription.js, obsolete following
8cde469501.

Differential Revision: https://code.wildfiregames.com/D1583
Accepted By: Vladislav
This was SVN commit r21859.

source/graphics/TextureManager.cpp

@@ -1,4 +1,4 @@
-/* Copyright (C) 2015 Wildfire Games.
+/* Copyright (C) 2018 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -316,6 +316,12 @@ public:
 		m_TextureConverter.ConvertTexture(texture, sourcePath, looseCachePath, settings);
 	}
 
+	bool TextureExists(const VfsPath& path) const
+	{
+		return m_VFS->GetFileInfo(m_CacheLoader.ArchiveCachePath(path), 0) == INFO::OK ||
+		       m_VFS->GetFileInfo(path, 0) == INFO::OK;
+	}
+
 	bool GenerateCachedTexture(const VfsPath& sourcePath, VfsPath& archiveCachePath)
 	{
 		archiveCachePath = m_CacheLoader.ArchiveCachePath(sourcePath);
@@ -653,6 +659,11 @@ CTexturePtr CTextureManager::CreateTexture(const CTextureProperties& props)
 	return m->CreateTexture(props);
 }
 
+bool CTextureManager::TextureExists(const VfsPath& path) const
+{
+	return m->TextureExists(path);
+}
+
 CTexturePtr CTextureManager::GetErrorTexture()
 {
 	return m->GetErrorTexture();