Reuse texture cache in ios_external_texture_gl (#11524)

In current implementation, external texture data flow is a
producer-consumer model. When painting external texture, it always asks
registered external texture object to produce new CVPixelBuffer, then
transforms it to texture. `MarkNewFrameAvailable` function is ignored.
This commit changes the dataflow. Now ios_external_texture_gl caches
previous opengl texture, if no new frame are available, it do not
`copyPixelBuffer` method, just uses cached opengl texture to draw.

This is a re-land of flutter/engine#9806, which was previously reverted
in flutter/engine#11522.

shell/platform/darwin/ios/ios_external_texture_gl.h

@@ -30,7 +30,9 @@ class IOSExternalTextureGL : public flutter::Texture {
   void CreateTextureFromPixelBuffer();
 
   void EnsureTextureCacheExists();
+  bool NeedUpdateTexture(bool freeze);
 
+  bool new_frame_ready_ = false;
   NSObject<FlutterTexture>* external_texture_;
   fml::CFRef<CVOpenGLESTextureCacheRef> cache_ref_;
   fml::CFRef<CVOpenGLESTextureRef> texture_ref_;

shell/platform/darwin/ios/ios_external_texture_gl.mm

@@ -53,17 +53,24 @@ void IOSExternalTextureGL::CreateTextureFromPixelBuffer() {
   }
 }
 
+bool IOSExternalTextureGL::NeedUpdateTexture(bool freeze) {
+  // Update texture if `texture_ref_` is reset to `nullptr` when GrContext
+  // is destroied or new frame is ready.
+  return (!freeze && new_frame_ready_) || !texture_ref_;
+}
+
 void IOSExternalTextureGL::Paint(SkCanvas& canvas,
                                  const SkRect& bounds,
                                  bool freeze,
                                  GrContext* context) {
   EnsureTextureCacheExists();
-  if (!freeze) {
+  if (NeedUpdateTexture(freeze)) {
     auto pixelBuffer = [external_texture_ copyPixelBuffer];
     if (pixelBuffer) {
       buffer_ref_.Reset(pixelBuffer);
     }
     CreateTextureFromPixelBuffer();
+    new_frame_ready_ = false;
   }
   if (!texture_ref_) {
     return;
@@ -93,6 +100,8 @@ void IOSExternalTextureGL::OnGrContextDestroyed() {
   cache_ref_.Reset(nullptr);
 }
 
-void IOSExternalTextureGL::MarkNewFrameAvailable() {}
+void IOSExternalTextureGL::MarkNewFrameAvailable() {
+  new_frame_ready_ = true;
+}
 
 }  // namespace flutter