move gotd fork into repo. (#111)

- update to latest telegram layer
- remove some references to fields in tg.Entities that don't exist in
the schema
- originally added here:
https://github.com/beeper/td/commit/820929062a2ba0104397bc01235ab58a9cff780e
  - referenced here
-
https://github.com/mautrix/telegramgo/commit/124f0967ed195b5a380c9bd02e170ada9710dde3
-
https://github.com/mautrix/telegramgo/commit/4205047aab2e0639217148b5d125bfaab668bd8e

pkg/gotd/telegram/downloader/downloader.go

@@ -0,0 +1,48 @@
+// Package downloader contains downloading files helpers.
+package downloader
+
+import (
+	"go.mau.fi/mautrix-telegram/pkg/gotd/bin"
+	"go.mau.fi/mautrix-telegram/pkg/gotd/tg"
+)
+
+// Downloader is Telegram file downloader.
+type Downloader struct {
+	partSize int
+	pool     *bin.Pool
+}
+
+const defaultPartSize = 512 * 1024 // 512 kb
+
+// NewDownloader creates new Downloader.
+func NewDownloader() *Downloader {
+	return new(Downloader).WithPartSize(defaultPartSize)
+}
+
+// WithPartSize sets chunk size.
+// Must be divisible by 4KB.
+//
+// See https://core.telegram.org/api/files#downloading-files.
+func (d *Downloader) WithPartSize(partSize int) *Downloader {
+	d.partSize = partSize
+	d.pool = bin.NewPool(partSize)
+	return d
+}
+
+// Download creates Builder for plain downloads.
+func (d *Downloader) Download(rpc Client, location tg.InputFileLocationClass) *Builder {
+	return newBuilder(d, master{
+		client:   rpc,
+		precise:  true,
+		allowCDN: false,
+		location: location,
+	})
+}
+
+// Web creates Builder for web files downloads.
+func (d *Downloader) Web(rpc Client, location tg.InputWebFileLocationClass) *Builder {
+	return newBuilder(d, web{
+		client:   rpc,
+		location: location,
+	})
+}