This fixes a crash (preceeded by Vulkan validation errors) when running
the following test:

--gtest_filter=dEQP.KHR_GLES31/core_texture_storage_multisample_FunctionalTests_blitting_multisampled_depth_attachment* --use-angle=vulkan

The problem is that GraphicsPipelineDesc::updateRasterizationSamples()
is rarely called (e.g. glBindFramebuffer).  The above dEQP test changes
between draw attachments that do and don't have multisampling enabled.
The number of samples is set (the first time) to have multisampling
enabled, and never changed, even though other attachments don't have
multisampling.

FramebufferVk::syncState() now calls updateRasterizationSamples().  That
fix uncovered a latent problem: when an application destroys all
framebuffers (including the draw framebuffer), the ContextVk still
points at the old draw framebuffer's FramebufferVk (now freed).  This
is fixed with new plubming to clean-up ContextVk::mDrawFramebuffer.

Bug: angleproject:4240
Change-Id: I151ec40ab821efc07c26323c34275c9165ad2ed5
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1983393Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Ian Elliott <ianelliott@google.com>