Replace rx::Renderer with rx::ContextImpl.

Previously Context had no Impl class, but had a special relationship
with the instanced Renderer class. Having a ContextImpl backing every
Context will allow new designs to enable things like multithreading
(where each ContextImpl stores a Context-specific device) or non-
virtual Contexts on Android or other platforms where it is more
efficient.

A large refactoring patch that touches every back-end.

BUG=angleproject:1363

Change-Id: Icb73a7d37447f08a664eeb499a310ba05d71a57e
Reviewed-on: https://chromium-review.googlesource.com/342052
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
diff --git a/src/libANGLE/renderer/ProgramImpl.h b/src/libANGLE/renderer/ProgramImpl.h
index ee78b1c..12c4a3c 100644
--- a/src/libANGLE/renderer/ProgramImpl.h
+++ b/src/libANGLE/renderer/ProgramImpl.h
@@ -14,10 +14,14 @@
 #include "libANGLE/Constants.h"
 #include "libANGLE/Program.h"
 #include "libANGLE/Shader.h"
-#include "libANGLE/renderer/Renderer.h"
 
 #include <map>
 
+namespace sh
+{
+struct BlockMemberInfo;
+}
+
 namespace rx
 {