Reduce load delay with many recent http files.

If we have data for them, we can start up fairly quickly.

Core/FileLoaders/DiskCachingFileLoader.cpp

@@ -54,6 +54,11 @@ DiskCachingFileLoader::~DiskCachingFileLoader() {
 }
 
 bool DiskCachingFileLoader::Exists() {
+	if (cache_->HasData()) {
+		// It may require a slow operation to check - if we have data, let's say yes.
+		// This helps initial load, since we check each recent file for existence.
+		return true;
+	}
 	return backend_->Exists();
 }
 
@@ -718,6 +723,19 @@ void DiskCachingFileLoaderCache::CloseFileHandle() {
 	fd_ = 0;
 }
 
+bool DiskCachingFileLoaderCache::HasData() const {
+	if (!f_) {
+		return false;
+	}
+
+	for (size_t i = 0; i < blockIndexLookup_.size(); ++i) {
+		if (blockIndexLookup_[i] != INVALID_INDEX) {
+			return true;
+		}
+	}
+	return false;
+}
+
 u64 DiskCachingFileLoaderCache::FreeDiskSpace() {
 	std::string dir = cacheDir_;
 	if (dir.empty()) {

Core/FileLoaders/DiskCachingFileLoader.h

@@ -89,6 +89,8 @@ public:
 	// Guaranteed to read at least one block into the cache.
 	size_t SaveIntoCache(FileLoader *backend, s64 pos, size_t bytes, void *data);
 
+	bool HasData() const;
+
 private:
 	void InitCache(const std::string &path);
 	void ShutdownCache();