Vulkan: Implement simple render-to-texture.

This was mostly working already, just needed to set up a few entry
points.

BUG=angleproject:2200

Change-Id: I9c13d6d4dd42f23c69a58e42e07e3e28877671a1
Reviewed-on: https://chromium-review.googlesource.com/734237
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
diff --git a/src/libANGLE/renderer/vulkan/FramebufferVk.cpp b/src/libANGLE/renderer/vulkan/FramebufferVk.cpp
index 9c8d155..2931e63 100644
--- a/src/libANGLE/renderer/vulkan/FramebufferVk.cpp
+++ b/src/libANGLE/renderer/vulkan/FramebufferVk.cpp
@@ -359,8 +359,7 @@
 
 bool FramebufferVk::checkStatus(const gl::Context *context) const
 {
-    UNIMPLEMENTED();
-    return bool();
+    return true;
 }
 
 void FramebufferVk::syncState(const gl::Context *context,
@@ -618,7 +617,6 @@
     // Updated the cached image layout of the attachments in this FBO.
     // For a default FBO, we need to call through to the WindowSurfaceVk
     // TODO(jmadill): Iterate over all attachments.
-    ASSERT(mBackbuffer);
     RenderTargetVk *renderTarget = nullptr;
     ANGLE_TRY(mState.getFirstColorAttachment()->getRenderTarget(context, &renderTarget));
     renderTarget->image->updateLayout(VK_IMAGE_LAYOUT_COLOR_ATTACHMENT_OPTIMAL);