Windows Executable Cross-Compile with MinGW

* Added Windows build instructions to Building.md and added a
  "make mingw" command to cross-compile the Windows binary into
  the bin/ folder.
* Fix a bug in the Wallpaper texture loader where it would error out
  when caching textures to disk the first time.

Building.md

@@ -29,6 +29,49 @@ sudo dnf -y install golang SDL2-devel SDL2_ttf-devel
 sudo apt -y install golang libsdl2-dev libsdl2-ttf-dev
 ```
 
-## Windows
+## Windows Cross-Compile from Linux
 
-TBD.
+Install the Mingw C compiler:
+
+```bash
+# Fedora
+sudo dnf -y install mingw64-gcc  # for 64-bit
+sudo dnf -y install mingw32-gcc  # for 32-bit
+
+# Arch Linux
+pacman -S mingw-w64
+```
+
+Download the SDL2 Mingw development libraries [here](https://libsdl.org/download-2.0.php)
+and SDL2_TTF from [here](https://www.libsdl.org/projects/).
+
+Extract each and copy their library folder into the mingw path.
+
+```bash
+# e.g. /usr/x86_64-w64-mingw32 is usually the correct path, verify on e.g.
+# Fedora with `rpm -ql mingw64-filesystem`
+tar -xzvf SDL2_ttf-devel-2.0.15-mingw.tar.gz
+cd SDL_ttf-2.0.15
+sudo cp -r x86_64-w64-mingw32 /usr
+```
+
+Make and set permissions for Go to download the standard library for Windows:
+
+```bash
+mkdir /usr/lib/golang/pkg/windows_amd64
+chown your_username /usr/lib/golang/pkg/windows_amd64
+```
+
+And run `make mingw` to build the Windows binary.
+
+### Windows DLLs
+
+For the .exe to run it will need SDL2.dll and such.
+
+```
+# SDL2.dll and SDL2_ttf.dll
+cp /usr/x86_64-w64-mingw32/bin/SDL*.dll bin/
+```
+
+SDL2_ttf requires libfreetype, you can get its DLL here:
+https://github.com/ubawurinna/freetype-windows-binaries

Makefile

@@ -19,6 +19,18 @@ build:
 	go build $(LDFLAGS) -i -o bin/doodle cmd/doodle/main.go
 	go build $(LDFLAGS) -i -o bin/doodad cmd/doodad/main.go
 
+# `make mingw` to cross-compile a Windows binary with mingw.
+.PHONY: mingw
+mingw:
+	env CGO_ENABLED="1" CC="/usr/bin/x86_64-w64-mingw32-gcc" \
+		GOOS="windows" CGO_LDFLAGS="-lmingw32 -lSDL2" CGO_CFLAGS="-D_REENTRANT" \
+		go build $(LDFLAGS) -i -o bin/doodle.exe cmd/doodle/main.go
+		env CGO_ENABLED="1" CC="/usr/bin/x86_64-w64-mingw32-gcc" \
+			GOOS="windows" CGO_LDFLAGS="-lmingw32 -lSDL2" CGO_CFLAGS="-D_REENTRANT" \
+			go build $(LDFLAGS) -i -o bin/doodad.exe cmd/doodad/main.go
+
+
+
 # `make run` to run it in debug mode.
 .PHONY: run
 run:

pkg/wallpaper/texture.go

@@ -58,13 +58,17 @@ func texture(e render.Engine, img *image.RGBA, name string) (render.Texturer, er
 	if _, err := os.Stat(filename); os.IsNotExist(err) {
 		fh, err := os.Create(filename)
 		if err != nil {
-			return nil, fmt.Errorf("CornerTexture: %s", err.Error())
+			return nil, fmt.Errorf("texture(%s): %s", name, err.Error())
 		}
-		defer fh.Close()
 
 		err = bmp.Encode(fh, img)
 		if err != nil {
-			return nil, fmt.Errorf("CornerTexture: bmp.Encode: %s", err.Error())
+			return nil, fmt.Errorf("texture(%s): bmp.Encode: %s", name, err.Error())
+		}
+
+		err = fh.Close()
+		if err != nil {
+			return nil, fmt.Errorf("texture(%s): fh.Close: %s", name, err.Error())
 		}
 	}