Add a unit test for FlutterFrameBufferProvider (flutter#27591)

ci/licenses_golden/licenses_flutter

@@ -1194,6 +1194,7 @@ FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterExter
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterExternalTextureMetal.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterFrameBufferProvider.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterFrameBufferProvider.mm
+FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterFrameBufferProviderUnittests.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterGLCompositor.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterGLCompositor.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterGLCompositorUnittests.mm

shell/platform/darwin/macos/BUILD.gn

@@ -178,6 +178,7 @@ executable("flutter_desktop_darwin_unittests") {
     "framework/Source/FlutterEngineTest.mm",
     "framework/Source/FlutterEngineTestUtils.h",
     "framework/Source/FlutterEngineTestUtils.mm",
+    "framework/Source/FlutterFrameBufferProviderUnittests.mm",
     "framework/Source/FlutterGLCompositorUnittests.mm",
     "framework/Source/FlutterKeyboardManagerUnittests.mm",
     "framework/Source/FlutterMetalCompositorUnittests.mm",

shell/platform/darwin/macos/framework/Source/FlutterFrameBufferProviderUnittests.mm

@@ -0,0 +1,41 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#import <Foundation/Foundation.h>
+
+#import "flutter/shell/platform/darwin/macos/framework/Source/FlutterFrameBufferProvider.h"
+#import "flutter/testing/testing.h"
+
+#import <OpenGL/gl.h>
+
+namespace flutter::testing {
+
+TEST(FlutterFrameBufferProviderTest, TestCreate) {
+  NSOpenGLPixelFormatAttribute attributes[] = {
+      NSOpenGLPFAColorSize, 24, NSOpenGLPFAAlphaSize, 8, 0,
+  };
+  NSOpenGLPixelFormat* pixelFormat = [[NSOpenGLPixelFormat alloc] initWithAttributes:attributes];
+  NSOpenGLContext* context = [[NSOpenGLContext alloc] initWithFormat:pixelFormat shareContext:nil];
+
+  [context makeCurrentContext];
+  FlutterFrameBufferProvider* framebufferProvider =
+      [[FlutterFrameBufferProvider alloc] initWithOpenGLContext:context];
+
+  GLuint fbo = [framebufferProvider glFrameBufferId];
+  GLuint texture = [framebufferProvider glTextureId];
+
+  // Normally we'd back this using an IOSurface but for this test let's just create a TexImage2D
+  // with no backing data.
+  glBindTexture(GL_TEXTURE_RECTANGLE_ARB, texture);
+  glTexImage2D(GL_TEXTURE_RECTANGLE_ARB, 0, GL_RGBA, 1, 1, 0, GL_RGBA, GL_UNSIGNED_BYTE, nullptr);
+  glBindFramebuffer(GL_FRAMEBUFFER, fbo);
+  glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_RECTANGLE_ARB, texture,
+                         0);
+
+  GLenum status = glCheckFramebufferStatus(GL_FRAMEBUFFER);
+
+  EXPECT_TRUE(status == GL_FRAMEBUFFER_COMPLETE);
+}
+
+}  // flutter::testing