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/gl/FramebufferGL.cpp b/src/libANGLE/renderer/gl/FramebufferGL.cpp
index 009b75a..1b30710 100644
--- a/src/libANGLE/renderer/gl/FramebufferGL.cpp
+++ b/src/libANGLE/renderer/gl/FramebufferGL.cpp
@@ -417,7 +417,7 @@
                                 const gl::Rectangle &origArea,
                                 GLenum format,
                                 GLenum type,
-                                void *ptrOrOffset) const
+                                void *ptrOrOffset)
 {
     // Clip read area to framebuffer.
     const gl::Extents fbSize = getState().getReadAttachment()->getSize();