Add 'ebitengl' build tag to force to use GL

Fixes #958

cmd/ebitenmobile/gobind.go

@@ -79,6 +79,15 @@ func invokeOriginalGobind(lang string) (pkgName string, err error) {
 	return pkgs[0].Name, nil
 }
 
+func forceGL() bool {
+	for _, tag := range strings.Split(*tags, ",") {
+		if tag == "ebitengl" {
+			return true
+		}
+	}
+	return false
+}
+
 func run() error {
 	writeFile := func(filename string, content string) error {
 		if err := ioutil.WriteFile(filepath.Join(*outdir, filename), []byte(content), 0644); err != nil {
@@ -100,6 +109,12 @@ func run() error {
 			content = strings.ReplaceAll(content, "{{.PrefixUpper}}", prefixUpper)
 			content = strings.ReplaceAll(content, "{{.PrefixLower}}", prefixLower)
 			content = strings.ReplaceAll(content, "{{.JavaPkg}}", *javaPkg)
+
+			f := "0"
+			if forceGL() {
+				f = "1"
+			}
+			content = strings.ReplaceAll(content, "{{.ForceGL}}", f)
 			return content
 		}
 
@@ -135,7 +150,7 @@ const objcM = `// Code generated by ebitenmobile. DO NOT EDIT.
 
 #import <TargetConditionals.h>
 
-#if TARGET_IPHONE_SIMULATOR
+#if TARGET_IPHONE_SIMULATOR || {{.ForceGL}}
 #define EBITEN_METAL 0
 #else
 #define EBITEN_METAL 1

doc.go

@@ -51,4 +51,11 @@
 // In the API document, 'the main thread' means the goroutine in init(), main() and their callees without 'go'
 // statement. It is assured that 'the main thread' runs on the OS main thread. There are some Ebiten functions that
 // must be called on the main thread under some conditions (typically, before ebiten.Run is called).
+//
+// Build tags
+//
+// 'ebitendebug' outputs a log of graphics commands. This is useful to know what happens in Ebiten. In general, the
+// number of graphics commands affects the performance of your game.
+//
+// 'ebitengl' forces to use OpenGL in any environments.
 package ebiten

graphicsdriver_ios.go

@@ -13,6 +13,7 @@
 // limitations under the License.
 
 // +build darwin,ios,arm darwin,ios,arm64
+// +build !ebitengl
 
 package ebiten
 

graphicsdriver_mac.go

@@ -14,6 +14,7 @@
 
 // +build darwin,!ios
 // +build !js
+// +build !ebitengl
 
 package ebiten
 

graphicsdriver_opengl.go

@@ -12,9 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-// +build android darwin,ios,386 darwin,ios,amd64 freebsd js linux windows
-
-// As the Go playground tries to compile this with CGO_ENABLED=0 and GOOS=linux, check Cgo on build tags.
+// +build android darwin,ios,386 darwin,ios,amd64 freebsd js linux windows ebitengl
 
 package ebiten