ObjC: Add interface for injecting custom shaders in video views

This CL adds a way for external clients to inject their own OpenGL(ES)
shaders to RTCEAGLVideoView/RTCNSGLVideoView. The shader interface
takes textures as arguments, and not RTCVideoFrame, so that
implementations only has to deal with actual OpenGL rendering, and not
converting frames into textures.

This CL also moves the internal shader code around a bit. The current
RTCShader interface with the implementations RTCI420Shader and
RTCNativeNV12Shader are removed. RTCEAGLVideoView and RTCNSGLVideoView
will be responsible for uploading the frames to textures instead
using the helper classes RTCI420TextureCache and RTCNV12TextureCache.
They then call the shader implementation with these textures. The
rendering code that used to be in RTCI420Shader and RTCNativeNV12Shader
have been merged into one RTCDefaultShaderDelegate class.

BUG=webrtc:7473

Review-Url: https://codereview.webrtc.org/2869143002
Cr-Commit-Position: refs/heads/master@{#18326}

webrtc/sdk/objc/Framework/Classes/UI/RTCNSGLVideoView.m

@@ -18,7 +18,8 @@
 #import <CoreVideo/CVDisplayLink.h>
 #import <OpenGL/gl3.h>
 
-#import "RTCShader+Private.h"
+#import "RTCDefaultShader.h"
+#import "RTCI420TextureCache.h"
 #import "WebRTC/RTCLogging.h"
 #import "WebRTC/RTCVideoFrame.h"
 
@@ -26,7 +27,7 @@
 // |videoFrame| is set when we receive a frame from a worker thread and is read
 // from the display link callback so atomicity is required.
 @property(atomic, strong) RTCVideoFrame *videoFrame;
-@property(atomic, strong) id<RTCShader> i420Shader;
+@property(atomic, strong) RTCI420TextureCache *i420TextureCache;
 
 - (void)drawFrame;
 @end
@@ -45,11 +46,25 @@ static CVReturn OnDisplayLinkFired(CVDisplayLinkRef displayLink,
 @implementation RTCNSGLVideoView {
   CVDisplayLinkRef _displayLink;
   RTCVideoFrame *_lastDrawnFrame;
+  id<RTCVideoViewShading> _shader;
 }
 
 @synthesize delegate = _delegate;
 @synthesize videoFrame = _videoFrame;
-@synthesize i420Shader = _i420Shader;
+@synthesize i420TextureCache = _i420TextureCache;
+
+- (instancetype)initWithFrame:(NSRect)frame pixelFormat:(NSOpenGLPixelFormat *)format {
+  return [self initWithFrame:frame pixelFormat:format shader:[[RTCDefaultShader alloc] init]];
+}
+
+- (instancetype)initWithFrame:(NSRect)frame
+                  pixelFormat:(NSOpenGLPixelFormat *)format
+                       shader:(id<RTCVideoViewShading>)shader {
+  if (self = [super initWithFrame:frame pixelFormat:format]) {
+    _shader = shader;
+  }
+  return self;
+}
 
 - (void)dealloc {
   [self teardownDisplayLink];
@@ -85,7 +100,7 @@ static CVReturn OnDisplayLinkFired(CVDisplayLinkRef displayLink,
 
 - (void)clearGLContext {
   [self ensureGLContext];
-  self.i420Shader = nil;
+  self.i420TextureCache = nil;
   [super clearGLContext];
 }
 
@@ -118,15 +133,20 @@ static CVReturn OnDisplayLinkFired(CVDisplayLinkRef displayLink,
   glClear(GL_COLOR_BUFFER_BIT);
 
   // Rendering native CVPixelBuffer is not supported on OS X.
+  // TODO(magjed): Add support for NV12 texture cache on OS X.
   frame = [frame newI420VideoFrame];
-  if (!self.i420Shader) {
-    self.i420Shader = [[RTCI420Shader alloc] initWithContext:context];
+  if (!self.i420TextureCache) {
+    self.i420TextureCache = [[RTCI420TextureCache alloc] initWithContext:context];
   }
-  if (self.i420Shader && [self.i420Shader drawFrame:frame]) {
+  RTCI420TextureCache *i420TextureCache = self.i420TextureCache;
+  if (i420TextureCache) {
+    [i420TextureCache uploadFrameToTextures:frame];
+    [_shader applyShadingForFrameWithRotation:frame.rotation
+                                       yPlane:i420TextureCache.yTexture
+                                       uPlane:i420TextureCache.uTexture
+                                       vPlane:i420TextureCache.vTexture];
     [context flushBuffer];
     _lastDrawnFrame = frame;
-  } else {
-    RTCLog(@"Failed to draw frame.");
   }
   CGLUnlockContext([context CGLContextObj]);
 }