Vulkan: Fix readPixels to support sub image reads correctly

Also, do not return InvalidOperation from flush in order to be able to support
running dEQP tests  for Vulkan.

Bug:angleproject:2346

Change-Id: I12f3bd115034e044fb0fa2d94687321ede4a1e9d
Reviewed-on: https://chromium-review.googlesource.com/955889
Commit-Queue: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
diff --git a/src/libANGLE/renderer/vulkan/ContextVk.cpp b/src/libANGLE/renderer/vulkan/ContextVk.cpp
index 6a03392..213cad6 100644
--- a/src/libANGLE/renderer/vulkan/ContextVk.cpp
+++ b/src/libANGLE/renderer/vulkan/ContextVk.cpp
@@ -128,7 +128,11 @@
 {
     // TODO(jmadill): Flush will need to insert a semaphore for the next flush to wait on.
     UNIMPLEMENTED();
-    return gl::InternalError();
+
+    // dEQP tests rely on having no errors thrown at the end of the test and they always call
+    // flush at the end of the their tests. Just returning NoError until we implement flush
+    // allow us to work on enabling many tests in the meantime.
+    return gl::NoError();
 }
 
 gl::Error ContextVk::finish(const gl::Context *context)