Split pixelBuffer from pack/unpack state

This will refactor will help use packed enums for buffer targets.

BUG=angleproject:2169

Change-Id: Ie7ed3e105f89457c67027e6598d7e29503ad355c
Reviewed-on: https://chromium-review.googlesource.com/745181
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
diff --git a/src/libANGLE/renderer/vulkan/FramebufferVk.cpp b/src/libANGLE/renderer/vulkan/FramebufferVk.cpp
index b4a6c98..cfa26a6 100644
--- a/src/libANGLE/renderer/vulkan/FramebufferVk.cpp
+++ b/src/libANGLE/renderer/vulkan/FramebufferVk.cpp
@@ -327,7 +327,8 @@
     params.format      = format;
     params.type        = type;
     params.outputPitch = inputPitch;
-    params.pack.copyFrom(context, glState.getPackState());
+    params.packBuffer  = glState.getTargetBuffer(GL_PIXEL_UNPACK_BUFFER);
+    params.pack        = glState.getPackState();
 
     PackPixels(params, angleFormat, inputPitch, mapPointer, reinterpret_cast<uint8_t *>(pixels));