Move Framebuffer clearing from Renderer to the Framebuffer object.

BUG=angle:841

Change-Id: I95c9cbdc2d1c99731e19c48e18117358d22b9e94
Reviewed-on: https://chromium-review.googlesource.com/232690
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>
Move ReadPixels to the Framebuffer object and Impl.

BUG=angle:841

Change-Id: I71deac9e755b5dfa010596cd1f8a213c24d895bf
Reviewed-on: https://chromium-review.googlesource.com/232691
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>
Move blit to the Framebuffer object and Impl.

BUG=angle:841

Change-Id: I482e53a90606d9d6b105c7006234215d51ab1a6b
Reviewed-on: https://chromium-review.googlesource.com/232692
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>