iOS: Add support for rendering native CVPixelBuffers directly

This CL adds support in RTCEAGLVideoView for rendering CVPixelBuffers as
OpenGL ES textures directly, compared to the current code that first
converts the CVPixelBuffers to I420, and then reuploads them as
textures. This is only supported on iOS with the use of a
CVOpenGLESTextureCache.

The I420 rendering and native rendering are separated in two different
implementations of a simple shader interface:
@protocol Shader
- (BOOL)drawFrame:(RTCVideoFrame*)frame;
@end
GL resources are allocated when the shader is instantiated and released
when the shader is destroyed. RTCEAGLVideoView will lazily instantiate
the necessary shader when it receives the first frame of that kind. This
is primarily done to avoid allocating GL resources for both I420 and
native rendering.

Some other changes are:
 - Print GL shader compilation errors.
 - Remove updateTextureSizesForFrame() function. The textures will
   resize automatically anyway when the texture data is uploaded with
   glTexImage2D().

patch from issue 2154243002 at patchset 140001 (http://crrev.com/2154243002#ps140001)
Continuing magjed@'s work since he is OOO this week.

BUG=

Review-Url: https://codereview.webrtc.org/2202823004
Cr-Commit-Position: refs/heads/master@{#13668}

webrtc/sdk/objc/Framework/Classes/RTCEAGLVideoView.m

@@ -210,10 +210,12 @@
 }
 
 - (void)renderFrame:(RTCVideoFrame *)frame {
+#if !TARGET_OS_IPHONE
   // Generate the i420 frame on video send thread instead of main thread.
   // TODO(tkchin): Remove this once RTCEAGLVideoView supports uploading
-  // CVPixelBuffer textures.
+  // CVPixelBuffer textures on OSX.
   [frame convertBufferIfNeeded];
+#endif
   self.videoFrame = frame;
 }