graphicsdriver/opengl: Enable dFdx for mobiles

Fixes #1404

internal/graphicsdriver/opengl/shader_desktop.go

@@ -12,7 +12,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// +build !android
 // +build !js
+// +build !ios
 
 package opengl
 

internal/graphicsdriver/opengl/shader_js.go

@@ -22,5 +22,5 @@ func glslVersion() glsl.GLSLVersion {
 	if isWebGL2Available {
 		return glsl.GLSLVersionES300
 	}
-	return glsl.GLSLVersionWebGL1
+	return glsl.GLSLVersionES100
 }

internal/graphicsdriver/opengl/shader_mobile.go

@@ -0,0 +1,25 @@
+// Copyright 2020 The Ebiten Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build android ios
+
+package opengl
+
+import (
+	"github.com/hajimehoshi/ebiten/v2/internal/shaderir/glsl"
+)
+
+func glslVersion() glsl.GLSLVersion {
+	return glsl.GLSLVersionES100
+}

internal/shaderir/glsl/glsl.go

@@ -28,7 +28,7 @@ type GLSLVersion int
 
 const (
 	GLSLVersionDefault GLSLVersion = iota
-	GLSLVersionWebGL1
+	GLSLVersionES100
 	GLSLVersionES300
 )
 
@@ -42,7 +42,7 @@ func VertexPrelude(version GLSLVersion) string {
 func FragmentPrelude(version GLSLVersion) string {
 	var prefix string
 	switch version {
-	case GLSLVersionWebGL1:
+	case GLSLVersionES100:
 		prefix = `#extension GL_OES_standard_derivatives : enable` + "\n\n"
 	case GLSLVersionES300:
 		prefix = `#version 300 es` + "\n\n"