Add chdir option for Flatpak

* The --chdir CLI option to doodle will set a working directory for the
  game to switch to on startup. Flatpak builds place the files at
  /app/share/doodle where the ./rtp and ./guidebook files are relative
  to and this allows the game to find its sound effects and such.

Building.md

@@ -1,6 +1,7 @@
 # Building Doodle
 
 * [Linux](#linux)
+* [Flatpak for Linux](#flatpak-for-linux)
 * [Windows Cross-Compile from Linux](#windows-cross-compile-from-linux)
 * [Mac OS](#mac os)
 
@@ -16,6 +17,14 @@ You'll need the following git repositories:
 The [doodle-docker](https://git.kirsle.net/apps/doodle-docker) repo will
 be more up-to-date than the instructions below.
 
+The `bootstrap.py` script should attempt to git clone most dependencies into a
+folder named `deps/` relative to the root of the doodle repo. For example,
+`deps/render` would be a git clone of `git.kirsle.net/go/render`. By updating
+Doodle's go.mod file to replace these dependencies, you can produce a fully
+self-contained source directory that can fully build the app. The
+[Flatpak repo](#flatpak-for-linux) makes use of this. Below is how you would
+do this process very manually:
+
 ```bash
 # Clone all the repos down to your project folder
 git clone git@git.kirsle.net:apps/doodle-rtp rtp
@@ -147,6 +156,10 @@ sudo dnf -y install golang SDL2-devel SDL2_ttf-devel SDL2_mixer-devel
 sudo apt -y install golang libsdl2-dev libsdl2-ttf-dev libsdl2-mixer-devel
 ```
 
+## Flatpak for Linux
+
+The repo for this is at <https://code.sketchymaze.com/game/flatpak>.
+
 ## Mac OS
 
 ```bash

cmd/doodle/main.go

@@ -63,6 +63,10 @@ func main() {
 			Aliases: []string{"d"},
 			Usage:   "enable debug level logging",
 		},
+		&cli.StringFlag{
+			Name:  "chdir",
+			Usage: "working directory for the game's runtime package",
+		},
 		&cli.BoolFlag{
 			Name:    "edit",
 			Aliases: []string{"e"},
@@ -84,6 +88,14 @@ func main() {
 	}
 
 	app.Action = func(c *cli.Context) error {
+		// --chdir into a different working directory? e.g. for Flatpak especially.
+		if doodlePath := c.String("chdir"); doodlePath != "" {
+			if err := os.Chdir(doodlePath); err != nil {
+				log.Error("--chdir: couldn't enter '%s': %s", doodlePath, err)
+				return err
+			}
+		}
+
 		var filename string
 		if c.NArg() > 0 {
 			filename = c.Args().Get(0)
@@ -143,6 +155,10 @@ func main() {
 			engine.Maximize()
 		}
 
+		// Log what Doodle thinks its working directory is, for debugging.
+		pwd, _ := os.Getwd()
+		log.Debug("PWD: %s", pwd)
+
 		game.Run()
 		return nil
 	}