ObjC: Split out I420 texture uploading into separate class

This CL splits out the code in RTCI420Shader.mm for uploading byte
buffers to textures into its own class RTCI420TextureCache. The purpose
is to prepare for allowing clients to inject their own shaders.
RTCI420TextureCache will be used in the generic code, while the actual
shaders in RTCI420Shader will be customizable by the client.

BUG=webrtc:7473

Review-Url: https://codereview.webrtc.org/2842453002
Cr-Commit-Position: refs/heads/master@{#17882}

webrtc/sdk/objc/Framework/Classes/RTCOpenGLDefines.h

@@ -8,6 +8,8 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
+#import <Foundation/Foundation.h>
+
 #if TARGET_OS_IPHONE
 #define RTC_PIXEL_FORMAT GL_LUMINANCE
 #define SHADER_VERSION