x/mobile/gl: use EGL to determine the version of OpenGL ES

Author: BenLubar

gl/egl.go

@@ -0,0 +1,49 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build linux
+// +build linux
+
+package gl
+
+// #include <EGL/egl.h>
+// #cgo LDFLAGS: -lEGL
+import "C"
+
+func init() {
+	display := C.eglGetDisplay(C.EGL_DEFAULT_DISPLAY)
+
+	C.eglInitialize(display, nil, nil)
+
+	attributes := []C.EGLint{
+		C.EGL_RED_SIZE, 1,
+		C.EGL_GREEN_SIZE, 1,
+		C.EGL_BLUE_SIZE, 1,
+		C.EGL_NONE,
+	}
+
+	var (
+		config C.EGLConfig
+		count  C.EGLint
+	)
+	C.eglChooseConfig(display, &attributes[0], &config, 1, &count)
+
+	C.eglBindAPI(C.EGL_OPENGL_ES_API)
+
+	attributes = []C.EGLint{
+		C.EGL_CONTEXT_CLIENT_VERSION, 3,
+		C.EGL_NONE,
+	}
+
+	context := C.eglCreateContext(display, config, C.EGLContext(C.EGL_NO_CONTEXT), &attributes[0])
+	if context == nil {
+		version = "GL_ES_2_0"
+
+		return
+	}
+
+	C.eglDestroyContext(display, context)
+
+	version = "GL_ES_3_0"
+}

gl/work.go

@@ -84,16 +84,19 @@ func NewContext() (Context, Worker) {
 		work:          make(chan call, workbufLen),
 		retvalue:      make(chan C.uintptr_t),
 	}
-	if C.GLES_VERSION == "GL_ES_2_0" {
+	if version == "GL_ES_2_0" {
 		return glctx, glctx
 	}
 	return context3{glctx}, glctx
 }
 
+// version is determined at runtime on platforms that support EGL.
+var version = C.GLES_VERSION
+
 // Version returns a GL ES version string, either "GL_ES_2_0" or "GL_ES_3_0".
 // Future versions of the gl package may return "GL_ES_3_1".
 func Version() string {
-	return C.GLES_VERSION
+	return version
 }
 
 func (ctx *context) enqueue(c call) uintptr {
@@ -148,11 +151,8 @@ func (ctx *context) DoWork() {
 	}
 }
 
-func init() {
-	if unsafe.Sizeof(C.GLint(0)) != unsafe.Sizeof(int32(0)) {
-		panic("GLint is not an int32")
-	}
-}
+// GLint should be the same size as int32
+var _ [unsafe.Sizeof(C.GLint(0))]struct{} = [unsafe.Sizeof(int32(0))]struct{}{}
 
 // cString creates C string off the Go heap.
 // ret is a *char.