Vulkan: Only init RenderPass once per frame.

This saves some time spent in the driver, by making multiple draw
calls happen inside a single RenderPass.

This also makes the ReadPixels impl method non-const. I think in
the future we should avoid making const Impl methods unless they're
totally trivial.

BUG=angleproject:1898

Change-Id: I39172270a2f7dc5c1c2e3d4cc50af3bac8a29fa1
Reviewed-on: https://chromium-review.googlesource.com/672148
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/libANGLE/renderer/d3d/FramebufferD3D.cpp b/src/libANGLE/renderer/d3d/FramebufferD3D.cpp
index 0d1a38e..b7ce556 100644
--- a/src/libANGLE/renderer/d3d/FramebufferD3D.cpp
+++ b/src/libANGLE/renderer/d3d/FramebufferD3D.cpp
@@ -239,7 +239,7 @@
                                      const gl::Rectangle &origArea,
                                      GLenum format,
                                      GLenum type,
-                                     void *pixels) const
+                                     void *pixels)
 {
     // Clip read area to framebuffer.
     const gl::Extents fbSize = getState().getReadAttachment()->getSize();