internal/graphicsdriver/opengl/gl: dynamic function load for OpenGL ES

Updates #292
Updates #2450

internal/graphicsdriver/opengl/gl/default_android.go

@@ -12,12 +12,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-//go:build android || opengles
-
 package gl
 
-// #cgo !android pkg-config: glesv2
+// #cgo LDFLAGS: -lGLESv2
-// #cgo android  LDFLAGS:    -lGLESv2
 //
 // #include <GLES2/gl2.h>
 // #include <stdlib.h>

internal/graphicsdriver/opengl/gl/default_notpurego.go

@@ -1,7 +1,7 @@
 // SPDX-License-Identifier: MIT
 // SPDX-FileCopyrightText: 2014 Eric Woroshow
 
-//go:build !android && !darwin && !js && !windows && !opengles
+//go:build !android && !darwin && !js && !windows
 
 package gl
 

internal/graphicsdriver/opengl/gl/procaddr_gles.go

@@ -0,0 +1,46 @@
+// Copyright 2022 The Ebitengine 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.
+
+//go:build !android && !darwin && !js && !nintendosdk && !windows && opengles
+
+package gl
+
+// #cgo LDFLAGS: -ldl
+//
+// #include <dlfcn.h>
+// #include <stdlib.h>
+//
+// static void* getProcAddress(const char* name) {
+//   static void* libGLES;
+//   if (!libGLES) {
+//     libGLES = dlopen("libGLESv2.so", RTLD_NOW | RTLD_GLOBAL);
+//   }
+//   return dlsym(libGLES, name);
+// }
+import "C"
+
+import (
+	"strings"
+	"unsafe"
+)
+
+func getProcAddress(namea string) unsafe.Pointer {
+	const ext = "EXT"
+	if strings.HasSuffix(namea, ext) {
+		namea = namea[:len(namea)-len(ext)]
+	}
+	cname := C.CString(namea)
+	defer C.free(unsafe.Pointer(cname))
+	return C.getProcAddress(cname)
+}